U.S. patent number 9,238,848 [Application Number 13/636,993] was granted by the patent office on 2016-01-19 for high-strength steel sheet and method for producing same.
This patent grant is currently assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION. The grantee listed for this patent is Satoshi Hirose, Daisuke Maeda, Genichi Shigesato, Yoshihiro Suwa, Kenichi Yamamoto, Naoki Yoshinaga. Invention is credited to Satoshi Hirose, Daisuke Maeda, Genichi Shigesato, Yoshihiro Suwa, Kenichi Yamamoto, Naoki Yoshinaga.
United States Patent |
9,238,848 |
Suwa , et al. |
January 19, 2016 |
High-strength steel sheet and method for producing same
Abstract
A high-strength steel sheet includes, by mass %, C: 0.03% to
0.30%, Si: 0.08% to 2.1%, Mn: 0.5% to 4.0%, P: 0.05% or less, S:
0.0001% to 0.1%, N: 0.01% or less, acid-soluble Al: more than
0.004% and less than or equal to 2.0%, acid-soluble Ti: 0.0001% to
0.20%, at least one selected from Ce and La: 0.001% to 0.04% in
total, and a balance of iron and inevitable impurities, in which
[Ce], [La], [acid-soluble Al], and [S] satisfy
0.02.ltoreq.([Ce]+[La])/[acid-soluble Al]<0.25, and
0.4.ltoreq.([Ce]+[La])/[S].ltoreq.50 in a case in which the mass
percentages of Ce, La, acid-soluble Al, and S are defined to be
[Ce], [La], [acid-soluble Al], and [S], respectively, and a
microstructure includes 1% to 50% of martensite in terms of an area
ratio.
Inventors: |
Suwa; Yoshihiro (Tokyo,
JP), Yamamoto; Kenichi (Tokyo, JP), Maeda;
Daisuke (Tokyo, JP), Hirose; Satoshi (Tokyo,
JP), Shigesato; Genichi (Tokyo, JP),
Yoshinaga; Naoki (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Suwa; Yoshihiro
Yamamoto; Kenichi
Maeda; Daisuke
Hirose; Satoshi
Shigesato; Genichi
Yoshinaga; Naoki |
Tokyo
Tokyo
Tokyo
Tokyo
Tokyo
Tokyo |
N/A
N/A
N/A
N/A
N/A
N/A |
JP
JP
JP
JP
JP
JP |
|
|
Assignee: |
NIPPON STEEL & SUMITOMO METAL
CORPORATION (Tokyo, JP)
|
Family
ID: |
44914411 |
Appl.
No.: |
13/636,993 |
Filed: |
May 10, 2011 |
PCT
Filed: |
May 10, 2011 |
PCT No.: |
PCT/JP2011/060760 |
371(c)(1),(2),(4) Date: |
September 24, 2012 |
PCT
Pub. No.: |
WO2011/142356 |
PCT
Pub. Date: |
November 17, 2011 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20130008568 A1 |
Jan 10, 2013 |
|
Foreign Application Priority Data
|
|
|
|
|
May 10, 2010 [JP] |
|
|
2010-108431 |
Jun 11, 2010 [JP] |
|
|
2010-133709 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D
6/005 (20130101); C21D 8/0236 (20130101); C21D
8/0226 (20130101); C22C 38/02 (20130101); C22C
38/04 (20130101); C22C 38/06 (20130101); C22C
38/14 (20130101); C22C 38/005 (20130101); C22C
38/001 (20130101); C21D 2211/005 (20130101); C21D
2211/008 (20130101) |
Current International
Class: |
C21D
8/02 (20060101); C22C 38/00 (20060101); C22C
38/06 (20060101); C22C 38/04 (20060101); C22C
38/02 (20060101); C22C 38/14 (20060101); C21D
6/00 (20060101); C22F 1/10 (20060101) |
Field of
Search: |
;148/330,331,522,541 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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55-115923 |
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Sep 1980 |
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JP |
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59-96218 |
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Jun 1984 |
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JP |
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6-128688 |
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May 1994 |
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JP |
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2000-319756 |
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Nov 2000 |
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JP |
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2001-200331 |
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Jul 2001 |
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JP |
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2004-339593 |
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Dec 2004 |
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JP |
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2005-120436 |
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May 2005 |
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JP |
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2005-298896 |
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Oct 2005 |
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JP |
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2005-307301 |
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Nov 2005 |
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JP |
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1860249 |
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Nov 2006 |
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JP |
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2007-146280 |
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Jun 2007 |
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JP |
|
2007146280 |
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Jun 2007 |
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JP |
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2007-231369 |
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Sep 2007 |
|
JP |
|
2008-274336 |
|
Nov 2008 |
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JP |
|
2009-299137 |
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Dec 2009 |
|
JP |
|
20090018167 |
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Feb 2009 |
|
KR |
|
Other References
International Search Report issued in PCT/JP2011/060760, dated Jul.
26, 2011. cited by applicant .
Chinese Office Action dated Jan. 25, 2014, issued in Chinese Patent
Application No. 201180023000.8. cited by applicant .
Korean Notice of Decision to Grant, dated Aug. 4, 2014 for
Application No. 10-2012-7030367 with English language translation.
cited by applicant.
|
Primary Examiner: Olsen; Kaj K
Assistant Examiner: Polyansky; Alexander
Attorney, Agent or Firm: Birch, Stewart, Kolasch &
Birch, LLP
Claims
What is claimed is:
1. A high-strength steel sheet comprising, by mass %: C: 0.03% to
0.30%; Si: 0.08% to 2.1%; Mn: 0.5% to 4.0%; P: 0.05% or less; S:
0.0001% to 0.1%; N: 0.01% or less; acid-soluble Al: more than
0.020% and less than or equal to 2.0%; acid-soluble Ti: 0.0001% to
0.006%; at least one selected from Ce and La: 0.001% to 0.04% in
total; and a balance of iron and inevitable impurities, wherein
[Ce], [La], [acid-soluble Al], and [S] satisfy
0.02<([Ce]+[La])/[acid-soluble Al]<0.25, and 0.4
([Ce]+[La])/[S].ltoreq.50 in a case in which mass percentages of
Ce, La, acid-soluble Al, and S are defined to be [Ce], [La],
[acid-soluble Al], and [S], respectively, and a microstructure
thereof includes 1% to 50% of martensite in terms of an area
ratio.
2. The high-strength steel sheet according to claim 1 further
comprising, by mass %, at least one selected from a group
consisting of Cr: 0.001% to 2.0%, Ni: 0.001% to 2.0%, Cu: 0.001% to
2.0%, Nb: 0.001% to 0.2% V: 0.001% to 1.0%, W: 0.001% to 1.0%, Ca:
0.0001% to 0.01%, Mg: 0.0001% to 0.01%, Zr: 0.0001% to 0.2%, at
least one selected from Sc and lanthanoids of Pr through Lu:
0.0001% to 0.1% in total, As: 0.0001% to 0.5%, Co: 0.0001% to 1.0%,
Sn: 0.0001% to 0.2%, Pb: 0.0001% to 0.2%, Y: 0.0001% to 0.2%, and
Hf: 0.0001% to 0.2%.
3. The high-strength steel sheet according to claim 1, wherein
[Ce], [La], [acid-soluble Al], and [S] satisfy
0.02.ltoreq.([Ce]+[La])/[acid-soluble Al]<0.15.
4. The high-strength steel sheet according to claim 1, wherein
[Ce], [La], [acid-soluble Al], and [S] satisfy
0.02.ltoreq.([Ce]+[La])/[acid-soluble Al]<0.10.
5. The high-strength steel sheet according to claim 1, wherein a
number density of inclusions having an equivalent circle diameter
of 0.5 .mu.m to 2 .mu.m in the microstructure is 15
inclusions/mm.sup.2 or more.
6. The high-strength steel sheet according to claim 1, wherein, of
inclusions having an equivalent circle diameter of 1.0 .mu.m or
more in the microstructure, a number percentage of elongated
inclusions having an aspect ratio of 5 or more obtained by dividing
a long diameter by a short diameter is 20% or less.
7. The high-strength steel sheet according to claim 1, wherein, of
inclusions having an equivalent circle diameter of 1.0 .mu.m or
more in the microstructure, a number percentage of inclusions
having at least one of MnS, TiS, and (Mn, Ti)S precipitated to an
oxide or oxysulfide composed of at least one of Ce and La, and at
least one of O and S, or an oxide or oxysulfide composed of at
least one of Ce and La, at least one of Si and Ti, and at least one
of O and S is 10% or more.
8. The high-strength steel sheet according to claim 1, wherein a
volume number density of elongated inclusions having an equivalent
circle diameter of 1 .mu.m or more, and an aspect ratio of 5 or
more obtained by dividing a long diameter by a short diameter is
1.0.times.10.sup.4 inclusions/mm.sup.3 or less in the
microstructure.
9. The high-strength steel sheet according to claim 1, wherein, in
the microstructure, a volume number density of inclusions having at
least one of MnS, TiS, and (Mn, Ti)S precipitated in an oxide or
oxysulfide composed of at least one of Ce and La, and at least one
of O and S, or an oxide or oxysulfide composed of at least one of
Ce and La, at least one of Si and Ti, and at least one of 0 and S
is 1.0.times.10.sup.3 inclusions/1=.sup.3 or more.
10. The high-strength steel sheet according to claim 1, wherein
elongated inclusions having an equivalent circle diameter of 1
.mu.m or more, and an aspect ratio of 5 or more obtained by
dividing a long diameter by a short diameter are present in the
microstructure, and an average equivalent circle diameter of the
elongated inclusions is 10 .mu.m or less.
11. The high-strength steel sheet according to claim 1, wherein
inclusions having at least one of MnS, TiS, and (Mn, Ti)S
precipitated to an oxide or oxysulfide composed of at least one of
Ce and La, and at least one of O and S, or an oxide or oxysulfide
composed of at least one of Ce and La, at least one of Si and Ti,
and at least one of O and S are present in the microstructure, and
the inclusions include a total of 0.5 mass % to 95 mass % of at
least one of Ce and La in terms of an average composition.
12. The high-strength steel sheet according to claim 1, wherein an
average grain size in the microstructure is 10 .mu.m or less.
13. The high-strength steel sheet according to claim 1, wherein a
maximum hardness of martensite included in the microstructure is
600 Hv or less.
14. The high-strength steel sheet according to claim 1, wherein a
sheet thickness thereof is 0.5 mm to 20 mm.
15. The high-strength steel sheet according to claim 1, further
comprising a galvanized layer or a galvannealed layer on at least
one surface.
16. A method of manufacturing a high-strength steel sheet, the
method comprising: a first process in which a molten steel having
the chemical components according to claim 1 is subjected to a
continuous casting so as to be processed into a slab; a second
process in which a hot rolling is carried out on the slab in a
finishing temperature of 850.degree. C. to 970.degree. C., and a
steel sheet is manufactured; and a third process in which the steel
sheet is cooled to a cooling control temperature of 450.degree. C.
or lower at an average cooling rate of 10 to 100.degree. C./second,
and then coiled at a coiling temperature of 300.degree. C. to
450.degree. C., wherein a hot-rolled steel sheet is
manufactured.
17. The method of manufacturing the high-strength steel sheet
according to claim 16, wherein galvanizing or galvannealed is
carried out on at least one surface of the hot-rolled steel
sheet.
18. The method of manufacturing the high-strength steel sheet
according to claim 16, wherein galvanizing or galvannealed is
carried out on at least one surface of the cold-rolled steel
sheet.
19. The method of manufacturing the high-strength steel sheet
according to claim 1, wherein the slab is reheated to 1100.degree.
C. or higher after the first process and before the second process.
Description
FIELD OF THE INVENTION
The present invention relates to a high-strength steel sheet which
can be preferably mainly pressed and used in the underbody parts of
automobiles and the like and structural materials, and is excellent
in terms of hole expansion and ductility, and a method of producing
the same.
Priority is claimed on Japanese Patent Application No. 2010-108431,
filed May 10, 2010, and Japanese Patent Application No.
2010-133709, filed Jun. 11, 2010, the contents of which are
incorporated herein by reference.
DESCRIPTION OF RELATED ART
A steel sheet used for the structure of an automobile body needs to
have favorable formability and strength. As a high-strength steel
sheet having both formability and high strength, a steel sheet
composed of ferrite and martensite, a steel sheet composed of
ferrite and bainite, a steel sheet including retained austenite in
the microstructure, and the like are known.
The above complex microstructure steel sheets are disclosed in, for
example, Patent Citations 1 to 3. However, there is a demand for a
complex microstructure steel sheet having more favorable hole
expansion than in the conventional technique in order to meet
demands for an additional decrease in the weight of modern
automobiles and the capability of parts to have more complicated
shapes.
A complex microstructure steel sheet including martensite dispersed
in a ferrite matrix has a low yield ratio, a high tensile strength,
and an excellent elongation. However, in the complex microstructure
steel sheet, stress concentrates on the interfaces between ferrite
and martensite, cracks easily occur at the interfaces, and thus the
complex microstructure steel sheet has the disadvantage of poor
hole expansion.
In contrast to the above, Patent Citation 4 discloses a
high-strength hot-rolled steel sheet having excellent hole
expansion that are required for the recent wheel and underbody
member materials. In Patent Citation 4, the amount of C in the
steel sheet is decreased as much as possible so that a solid
solution-hardened or precipitation-hardened ferrite is included in
the steel sheet which includes bainite as a major part of the
microstructure at an appropriate volume fraction, the difference in
hardness between the ferrite and the bainite decreases, and
generation of coarse carbides is prevented.
In addition, Patent Citations 5 and 6 disclose methods in which
MnS-based coarse inclusions present in slabs are dispersed and
precipitated in a steel sheet as fine spherical inclusions which
include MnS so as to provide a high-strength steel sheet that is
excellent in terms of hole expansion without deteriorating fatigue
characteristics. In Patent Citation 5, deoxidation is carried out
by adding Ce and La without substantially adding Al, and fine MnS
is precipitated on fine and hard Ce oxides, La oxides, cerium
oxysulfides, and lanthanum oxysulfides, all of which are generated
by the deoxidation. In this technique, MnS does not elongate during
rolling, and therefore the MnS does not easily serve as a starting
point of cracking or crack propagation path, and the hole expansion
can be improved.
PATENT CITATION
[Patent Citation 1] Japanese Unexamined Patent Application, First
Publication No. H6-128688
[Patent Citation 2] Japanese Unexamined Patent Application, First
Publication No. 2000-319756
[Patent Citation 3] Japanese Unexamined Patent Application, First
Publication No. 2005-120436
[Patent Citation 4] Japanese Unexamined Patent Application, First
Publication No. 2001-200331
[Patent Citation 5] Japanese Unexamined Patent Application, First
Publication No. 2007-146280
[Patent Citation 6] Japanese Unexamined Patent Application, First
Publication No. 2008-274336
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
The high-strength hot-rolled steel sheet as disclosed in Patent
Citation 4, in which a major part of the microstructure is bainite,
and generation of coarse carbides is suppressed, exhibits excellent
hole expansion, but the ductility is poor compared to a steel sheet
mainly including ferrite and martensite. In addition, while
generation of coarse carbides is suppressed, it is still difficult
to prevent occurrence of cracks in a case in which a strict hole
expanding is carried out.
According to studies by the inventors, it was found that the above
disadvantages result from elongated sulfide-based inclusions mainly
including MnS in the steel sheet. When the steel sheet is
repeatedly deformed, internal defects are caused in the vicinity of
elongated coarse MnS-based inclusions that are present in and in
the vicinity of the surface layer of the steel sheet, the internal
defects propagate as cracks, and the fatigue characteristics
deteriorate. In addition, the elongated coarse MnS-based inclusions
become liable to serve as starting points of cracking during a hole
expanding.
Therefore, it is desirable to make MnS-based inclusions in steel
into a fine spherical shape while preventing the MnS-based
inclusions from being elongated as much as possible.
However, since Mn is an element that increases the strength of
materials together with C or Si, in a high-strength steel sheet, it
is common to set the concentration of Mn to a high percentage in
order to secure the strength. Furthermore, when a heavy treatment
for desulfurization is not carried out in a secondary refining, 50
ppm or more of S is included in steel. Therefore, generally, MnS is
present in slabs.
In addition, when the concentration of soluble Ti is increased in
order to improve stretch flangeability, the soluble Ti partially
bonds with coarse TiS and MnS so as to precipitate (Mn, Ti)S.
Since MnS-based inclusions (hereinafter, three inclusions of MnS,
TiS, and (Mn, Ti)S will be referred to as "MnS-based inclusions"
for convenience) are liable to deform when steels are hot-rolled or
cold-rolled, the MnS-based inclusions are elongated, which causes
degradation of hole expansion.
In contrast to Patent Citation 4, in Patent Citations 5 and 6,
since fine MnS-based inclusions are precipitated in slabs, and the
MnS-based inclusions are dispersed in the steel sheet as fine
spherical inclusions that do not easily serve as starting points of
cracking while not deforming during rolling, it is possible to
manufacture a hot-rolled steel sheet that is excellent in terms of
hole expansion.
However, in Patent Citation 5, since the steel sheet has a
microstructure mainly including bainite, sufficient ductility
cannot be expected compared to a steel sheet having microstructures
mainly including ferrite and martensite. In addition, in a steel
sheet having microstructures mainly including ferrite and
martensite, which are significantly different in hardness, hole
expansion are not significantly improved even when MnS-based
inclusions are finely precipitated using the techniques of Patent
Citations 5 and 6.
The present invention has been made to solve the problems of the
conventional techniques, and provides a complex microstructure type
high-strength steel sheet that is excellent in terms of hole
expansion and ductility, and a method of manufacturing the
same.
Methods for Solving the Problem
Hole expansion are a characteristic that is dependent on the
uniformity of the microstructure, and, in a multiphase steel sheet
mainly including ferrite and martensite having a large difference
in hardness in the microstructure, stress concentrates in the
interfaces between the ferrite and the martensite, and cracks are
liable to occur at the interfaces. Additionally, the hole expansion
are significantly deteriorated by sulfide-based inclusions in which
MnS and the like are elongated.
As a result of thorough studies, the inventors found that, when
chemical components and manufacturing conditions are adjusted so as
to prevent the hardness of a martensite phase (martensite) in a
multiphase steel sheet mainly including ferrite and martensite from
excessively increasing, and MnS-based inclusions are finely
precipitated through deoxidation by addition of Ce and La, hole
expansion can be significantly improved even in a steel sheet
having a microstructure in which ferrite and martensite are mainly
included, and completed the present invention.
Meanwhile, an example in which TiN is precipitated on fine and hard
Ce oxides, La oxides, cerium oxysulfides, and lanthanum oxysulfides
together with MnS-based inclusions was also observed, but it was
confirmed that such an example has little influence on hole
expansion and ductility.
Therefore, in the present invention, TiN will not be taken into
account as a partner of MnS-based inclusions.
The purports of the present invention are as follows:
(1) A high-strength steel sheet according to an aspect of the
present invention includes, by mass %, C, 0.03% to 0.30%, Si: 0.08%
to 2.1%, Mn: 0.5% to 4.0%, P: 0.05% or less, S: 0.0001% to 0.1%, N,
0.01% or less, acid-soluble Al: more than 0.004% and less than or
equal to 2.0%, acid-soluble Ti: 0.0001% to 0.20%, at least one
selected from Ce and La: 0.001% to 0.04% in total, and a balance of
iron and inevitable impurities, in which [Ce], [La], [acid-soluble
Al], and [S] satisfy 0.02.ltoreq.([Ce]+[La])/[acid-soluble
Al]<0.25, and 0.4.ltoreq.([Ce]+[La])/[S].ltoreq.50 in a case in
which the mass percentages of Ce, La, acid-soluble Al, and S are
defined to be [Ce], [La], [acid-soluble Al], and [S], respectively,
and the microstructure of the high-strength steel sheet includes 1%
to 50% of martensite in terms of an area ratio.
(2) The high-strength steel sheet according to the above (1) may
further include, by mass %, at least one selected from a group
consisting of Mo: 0.001% to 1.0%, Cr: 0.001% to 2.0%, Ni: 0.001% to
2.0%, Cu: 0.001% to 2.0%, B: 0.0001% to 0.005%, Nb: 0.001% to 0.2%,
V: 0.001% to 1.0%, W: 0.001% to 1.0%, Ca: 0.0001% to 0.01%, Mg:
0.0001% to 0.01%, Zr: 0.0001% to 0.2%, at least one selected from
Sc and lanthanoids of Pr through Lu: 0.0001% to 0.1%, As: 0.0001%
to 0.5%, Co: 0.0001% to 1.0%, Sn: 0.0001% to 0.2%, Pb: 0.0001% to
0.2%, Y: 0.0001% to 0.2%, and Hf: 0.0001% to 0.2%.
(3) In the high-strength steel sheet according to the above (1) or
(2), the amount of the acid-soluble Ti may be more than or equal to
0.0001% and less than 0.008%.
(4) In the high-strength steel sheet according to the above (1) or
(2), the amount of the acid-soluble Ti may be 0.008% to 0.20%.
(5) In the high-strength steel sheet according to the above (1) or
(2), [Ce], [La], [acid-soluble Al], and [S] may satisfy
0.02.ltoreq.([Ce]+[La])/[acid-soluble Al]<0.15.
(6) In the high-strength steel sheet according to the above (1) or
(2), [Ce], [La], [acid-soluble Al], and [S] may satisfy
0.02.ltoreq.([Ce]+[La])/[acid-soluble Al]<0.10.
(7) In the high-strength steel sheet according to the above (1) or
(2), the amount of acid-soluble Al may be more than 0.01% and less
than or equal to 2.0%.
(8) In the high-strength steel sheet according to the above (1) or
(2), the number density of inclusions having an equivalent circle
diameter of 0.5 .mu.m to 2 .mu.m in the microstructure may be 15
inclusions/mm.sup.2 or more.
(9) In the high-strength steel sheet according to the above (1) or
(2), of inclusions having an equivalent circle diameter of 1.0
.mu.m or more in the microstructure, the number percentage of
elongated inclusions having an aspect ratio of 5 or more obtained
by dividing the long diameter by the short diameter may be 20% or
less.
(10) In the high-strength steel sheet according to the above (1) or
(2), of inclusions having an equivalent circle diameter of 1.0
.mu.m or more in the microstructure, the number percentage of
inclusions having at least one of MnS, TiS, and (Mn, Ti)S
precipitated to an oxide or oxysulfide composed of at least one of
Ce and La, and at least one of O and S, or an oxide or oxysulfide
composed of at least one of Ce and La, at least one of Si and Ti,
and at least one of O and S may be 10% or more.
(11) In the high-strength steel sheet according to the above (1) or
(2), the volume number density of elongated inclusions having an
equivalent circle diameter of 1 .mu.m or more, and an aspect ratio
of 5 or more obtained by dividing the long diameter by the short
diameter may be 1.0.times.10.sup.4 inclusions/mm.sup.3 or less in
the steel structure.
(12) In the high-strength steel sheet according to the above (1) or
(2), in the microstructure, the volume number density of inclusions
having at least one of MnS, TiS, and (Mn, Ti)S precipitated to an
oxide or oxysulfide composed of at least one of Ce and La, and at
least one of O and S, or an oxide or oxysulfide composed of at
least one of Ce and La, at least one of Si and Ti, and at least one
of O and S may be 1.0.times.10.sup.3 inclusions/mm.sup.3 or
more.
(13) In the high-strength steel sheet according to the above (1) or
(2), elongated inclusions having an equivalent circle diameter of 1
.mu.m or more, and an aspect ratio of 5 or more obtained by
dividing the long diameter by the short diameter may be present in
the microstructure, and the average equivalent circle diameter of
the elongated inclusions may be 10 .mu.m or less.
(14) In the high-strength steel sheet according to the above (1) or
(2), inclusions having at least one of MnS, TiS, and (Mn, Ti)S
precipitated to an oxide or oxysulfide composed of at least one of
Ce and La, and at least one of O and S, or an oxide or oxysulfide
composed of at least one of Ce and La, at least one of Si and Ti,
and at least one of O and S may be present in the microstructure,
and the inclusions may include a total of 0.5 mass % to 95 mass %
of at least one of Ce and La in terms of an average
composition.
(15) In the high-strength steel sheet according to the above (1) or
(2), the average grain size in the microstructure may be 10 .mu.m
or less.
(16) In the high-strength steel sheet according to the above (1) or
(2), the maximum hardness of martensite included in the
microstructure may be 600 Hv or less.
(17) In the high-strength steel sheet according to the above (1) or
(2), the sheet thickness may be 0.5 mm to 20 mm.
(18) The high-strength steel sheet according to the above (1) or
(2) may further have a galvanized layer or a galvannealed layer on
at least one surface.
(19) A method of manufacturing a high-strength steel sheet
according to the aspect of the present invention includes a first
process in which molten steel having the chemical components
according to the above (1) or (2) is subjected to continuous
casting so as to be processed into a slab; a second process in
which hot rolling is carried out on the slab in a finishing
temperature of 850.degree. C. to 970.degree. C., and a steel sheet
is manufactured; and a third process in which the steel sheet is
cooled to a cooling control temperature of 650.degree. C. or lower
at an average cooling rate of 10.degree. C./second to 100.degree.
C./second, and then coiled at a coiling temperature of 300.degree.
C. to 650.degree. C.
(20) In the method of manufacturing the high-strength steel sheet
according to the above (19), in the third process, the cooling
control temperature may be 450.degree. C. or lower, the coiling
temperature may be 300.degree. C. to 450.degree. C., and a
hot-rolled steel sheet may be manufactured.
(21) The method of manufacturing the high-strength steel sheet
according to the above (19) may further include, after the third
process, a fourth process in which the steel sheet is pickled, and
cold rolling is carried out on the steel sheet at a reduction in
thickness of 40% or more; a fifth process in which the steel sheet
is annealed at a maximum temperature of 750.degree. C. to
900.degree. C.; a sixth process in which the steel sheet is cooled
to 450.degree. C. or lower at an average cooling rate of
0.1.degree. C./second to 200.degree. C./second; and a seventh
process in which the steel sheet is held in a temperature range of
300.degree. C. to 450.degree. C. for 1 second to 1000 seconds so as
to manufacture a cold-rolled steel sheet.
(22) In the method of manufacturing the high-strength steel sheet
according to the above (20), galvanizing or galvannealing may be
carried out on at least one surface of the hot-rolled steel
sheet.
(23) In the method of manufacturing the high-strength steel sheet
according to the above (21), galvanizing or galvannealing may be
carried out on at least one surface of the cold-rolled steel
sheet.
(24) In the method of manufacturing the high-strength steel sheet
according to the above (19), the slab may be reheated to
1100.degree. C. or higher after the first process and before the
second process.
Effects of the Invention
According to the present invention, it is possible to stably adjust
the chemical composition of molten steel, suppress generation of
coarse alumina inclusions, and precipitate sulfides in a slab
through fine MnS-based inclusions by controlling Al deoxidation and
deoxidation by addition of Ce and La. Since the fine MnS-based
inclusions are dispersed in the steel sheet as fine spherical
inclusions, do not deform during rolling, and do not easily serve
as starting points of cracking, it is possible to obtain a
high-strength steel sheet that is excellent in terms of hole
expansion and ductility.
Since the high-strength steel sheet according to the above (1) is a
multiphase steel sheet mainly including ferrite and martensite, the
ductility is excellent. In addition, in the high-strength steel
sheet according to the above (16), since the hardness of the
martensite phase is controlled, it is possible to further enhance
the effect of improving hole expansion by controlling the
morphology of inclusions. Furthermore, in the method of
manufacturing the high-strength steel sheet according to the above
(19), it is possible to manufacture a multiphase steel sheet mainly
including ferrite and martensite, in which fine MnS-based
inclusions are dispersed, that is, a high-strength steel sheet that
is excellent in terms of hole expansion and ductility.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view showing a relationship between the maximum
hardness and the hole expansion of a martensite phase.
FIG. 2 is a flowchart showing a method of manufacturing a
high-strength steel sheet according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the high-strength steel sheet of the present invention
will be described in detail. Hereinafter, mass % in chemical
components (chemical compositions) will be denoted simply by %.
Firstly, experiments that have been made until completion of the
present invention will be described.
Deoxidation by various amounts (chemical components in molten
steel) of Ce and La were carried out together with Al deoxidation
so as to manufacture slabs. The slabs were hot-rolled so as to
manufacture 3 mm hot-rolled steel sheets. Furthermore, the
hot-rolled steel sheets were pickled, then cold-rolled at a
reduction in thickness of 50%, and annealed under a variety of
annealing conditions so as to manufacture cold-rolled steel sheets.
The inventors provided the cold-rolled steel sheets for hole
expansion tests and tension tests, and investigated the number
densities, morphologies, and average chemical compositions of
inclusions in the steel sheets.
As a result of the above tests, it was found that, in molten steel
obtained by adding Si, then adding Al, then adding one or both of
Ce and La, and thereby carrying out deoxidation, in a case in which
([Ce]+[La])/[acid-soluble Al] and ([Ce]+[La])/[S] are in
predetermined ranges, the oxygen potential in the molten steel
abruptly decreases, the concentration of Al.sub.2O.sub.3 being
generated decreases, and a steel sheet that is excellent in terms
of hole expansion can be obtained. Here, [Ce], [La], [acid-soluble
Al], and [S] represent by mass % of Ce, La, acid-soluble Al, and S
that are included in steel, respectively (hereinafter, the same
expression as this description will be used).
The amount of increase in the hole expansion ratio of a cold-rolled
steel sheet to which one or both of Ce and La were added with
respect to the hole expansion ratio of a cold-rolled steel sheet to
which neither Ce nor La were added was varied by the hardness of a
martensite phase in the steel sheet, and the amount of increase
increased as the hardness decreased.
It could be confirmed that, when the maximum hardness of the
martensite phase was 600 Hv or less, the hole expansion were
improved more clearly by adding one or both of Ce and La. The
maximum hardness of the martensite phase refers to the maximum
value of micro Vickers hardness obtained by randomly pressing an
indenter with a load of 10 gf on a hard phase (other than a ferrite
phase) 50 times.
The cold-rolled steel sheet to which neither Ce nor La were added
(the steel sheet used to compare the hole expansion ratios) was
annealed under the same conditions so as to have the same tensile
strength as the cold-rolled steel sheet to which one or both of Ce
and La were added. In this case, it was confirmed that uniform
elongation of the cold-rolled steel sheet to which neither Ce nor
La were added and uniform elongation of the cold-rolled steel sheet
to which one or both of Ce and La were added were the same, and
deterioration of the ductility due to the addition of Ce and La was
not observed.
Meanwhile, in a microstructure that is substantially composed of
bainite, the hole expansion were significantly improved by addition
of Ce and La, but the ductility was small compared to the steel
sheet mainly including ferrite and martensite.
Reasons why the hole expansion were improved by addition of Ce and
La are considered to be as follows:
It is considered that, when Si is added to molten steel in
manufacturing a slab, SiO.sub.2 inclusions are formed, but the
SiO.sub.2 inclusions are reduced to Si by later addition of Al. Al
reduces SiO.sub.2 inclusions, and deoxidizes dissolved oxygen in
the molten steel so as to form Al.sub.2O.sub.3-based inclusions.
Some of Al.sub.2O.sub.3-based inclusions are removed though
floatation, and the rest of the Al.sub.2O.sub.3-based inclusions
remain in the molten steel.
After that, when Ce and La are added to the molten steel, a little
amount of Al.sub.2O.sub.3 remains, but the Al.sub.2O.sub.3-based
inclusions in the molten steel are reduced and decomposed, and fine
and hard Ce oxides, La oxides, cerium oxysulfides, and lanthanum
oxysulfides are formed by deoxidation using Ce and La.
When Al deoxidation is appropriately carried out based on the above
deoxidation, similarly to a case in which Al deoxidation is rarely
carried out, it is possible to precipitate MnS on the fine and hard
Ce oxides, La oxides, cerium oxysulfides, and lanthanum oxysulfides
which are formed by deoxidation by addition of Ce and La. As a
result, it is possible to suppress deformation of the precipitated
MnS during rolling, and therefore elongated coarse MnS in the steel
sheet can be significantly reduced, and the hole expansion can be
improved. Additionally, since it is also possible to further lower
the oxygen potential of the molten steel by Al deoxidation,
fluctuation in the chemical composition can be reduced.
Reasons why the degree of the hole expansion improved is varied by
the hardness of the martensite phase in steel sheets having the
same tensile strength and uniform elongation are considered to be
as follows.
Hole expansion are significantly affected by the local ductility of
a steel, and the most dominant factor in relation to hole expansion
is considered to be the difference in hardness between
microstructures (herein, between the martensite phase and the
ferrite phase). Other powerful dominant factors in relation to hole
expansion include the presence of nonmetallic inclusions, such as
MnS, and many publications report that voids are formed from the
inclusions as the starting points, grow, and link together such
that the steel breaks.
Therefore, if the hardness of the martensite phase is excessively
high, there are cases in which, even when the morphology of
inclusions are controlled by addition of Ce and La, and occurrence
of voids due to the inclusions is suppressed, stress concentrates
at the interfaces between ferrite and martensite, voids are formed
due to the difference in strength between the microstructures, and
thereby the steel may break.
The inventors newly found that, if the cooling conditions after hot
rolling in the case of a hot-rolled steel sheet and the annealing
conditions in the case of a cold-rolled steel sheet are
appropriately controlled, and the hardness of the martensite phase
is reduced, it is possible to further enhance the effect of
suppressing occurrence of voids by controlling the morphology of
the inclusions. In addition, the inventors found that a steel sheet
that is excellent in terms of ductility and hole expansion can be
obtained by securing a predetermined amount or more of martensite
in a microstructure mainly including ferrite and martensite, and
controlling the morphology of inclusions by adding Ce and La.
Meanwhile, it is possible to add Ti to the molten steel after Al is
added and before Ce and La are added. At this point in time, since
oxygen in the molten steel is already deoxidized by Al, the amount
of oxygen to be deoxidized by Ti is small. After that, due to Ce
and La that have been added to the molten steel,
Al.sub.2O.sub.3-based inclusions are reduced and decomposed, and
fine Ce oxides, La oxides, cerium oxysulfides, and lanthanum
oxysulfides are formed.
As described above, it is considered that, when complex deoxidation
is carried out by adding Al, Si, Ti, Ce, and La, a small amount of
Al.sub.2O.sub.3 remains, but fine and hard Ce oxides, La oxides,
cerium oxysulfides, lanthanum oxysulfides, and Ti oxides are mainly
formed.
During the complex deoxidation by addition of Al, Si, Ti, Ce, and
La, if the Al deoxidation is appropriately carried out based on the
deoxidation as described above, similarly to a case in which Al
deoxidation is rarely carried out, it is possible to precipitate
MnS, TiS, or (Mn, Ti)S on fine and hard oxides, such as Ce oxides,
La oxides, and Ti oxides, or fine and hard oxysulfides, such as
cerium oxysulfides and lanthanum oxysulfides. As a result, in a
case in which a predetermined amount or more of Ti is added to the
molten steel, the kinds of chemical elements included in inclusions
slightly vary, but a mechanism that suppresses elongation of
MnS-based inclusions was the same as in a case in which Ti is
rarely added.
Based on the finding obtained from experimental studies, the
inventors studied the chemical compositions, microstructures, and
manufacturing conditions of steel sheets as described below.
Firstly, a high-strength steel sheet according to an embodiment of
the present invention will be described.
Hereinafter, reasons why the chemical compositions are limited in
the high-strength steel sheet according to the embodiment of the
present invention will be described.
C is the most fundamental element that controls the hardenability
and strength of steel, which increases the hardness and thickness
of a layer hardened by quenching so as to improve the fatigue
strength. That is, C is an essential element for securing the
strength of a steel sheet. In order to form retained austenite and
low-temperature transformation phases that are necessary to obtain
a desired high-strength steel sheet, the concentration of C needs
to be 0.03% or more. When the concentration of C exceeds 0.30%,
formability and weldability deteriorate. Therefore, in order to
achieve a necessary strength and secure formability and
weldability, the concentration of C needs to be 0.30% or less. When
the balance between strength and formability is taken into account,
the concentration of C is preferably 0.05% to 0.20%, and more
preferably 0.10% to 0.15%.
Si is one major deoxidizing element. In addition, Si increases the
number of nucleation sites of austenite during heating for
quenching, and suppresses the grain growth of austenite so as to
refine the grain size in a layer hardened by quenching. In
addition, Si suppresses formation of carbides, and suppresses
degradation of grain boundary strength due to carbides.
Furthermore, Si is also effective for forming bainite, and plays a
critical role from the viewpoint of securing the overall
strength.
In order to develop the above effects, it is necessary to add 0.08%
or more of Si to steel. When the concentration of Si is too high,
even in a case in which Al deoxidation is sufficiently carried out,
the concentration of SiO.sub.2 in inclusions increases, and coarse
inclusions become liable to be formed. In addition, in this case,
toughness, ductility, and weldability deteriorate, and surface
decarburization and surface flaws increase so as to deteriorate
fatigue characteristics. Therefore, the upper limit of the
concentration of Si needs to be 2.1%. When the balance between
strength and other mechanical properties is taken into account, the
concentration of Si is preferably 0.10% to 1.5%, and more
preferably 0.12% to 1.0%.
Mn is a useful element for deoxidation in a steelmaking step, and
an effective element for increasing the strength of the steel sheet
together with C and Si. In order to obtain the above effect, the
concentration of Mn needs to be 0.5% or more. When more than 4.0%
of Mn is included in steel, ductility degrades due to segregation
of Mn and enhancement of solid solution strengthening. In addition,
since weldability and the toughness of a base metal deteriorate,
the upper limit of the concentration of Mn is 4.0%. When the
balance between strength and other mechanical properties is taken
into account, the concentration of Mn is preferably 1.0% to 3.0%,
and more preferably 1.2% to 2.5%.
P is useful in a case in which P is used as an element for
substitutional solid solution strengthening which is smaller than
an Fe atom. When the concentration of P in steel exceeds 0.05%,
there are cases in which P segregates at the grain boundaries of
austenite, the grain boundary strength degrades, and formability
may deteriorate. Therefore, the upper limit of the concentration of
P is 0.05%. When solid solution strengthening is not required, it
is not necessary to add P to steel, and therefore the lower limit
of the concentration of P includes 0%. Meanwhile, for example, the
lower limit of the concentration of P may be 0.0001% in
consideration of the concentration of P included as an
impurity.
N is an element that is inevitably incorporated into steel since
nitrogen in the air is trapped into molten steel during treating
molten steel. N has an action of forming nitrides with chemical
elements, such as Al and Ti, so as to promote refining of the
microstructure in the base metal. However, when the concentration
of N exceeds 0.01%, N forms coarse precipitates with chemical
elements, such as Al and Ti, and hole expansion deteriorate.
Therefore, the upper limit of the concentration of N is 0.01%. On
the other hand, when the concentration of N is reduced to less than
0.0005%, the cost increases, and therefore the lower limit of the
concentration of N may be 0.0005% from the viewpoint of industrial
feasibility.
S is included in the steel sheet as an impurity, and liable to
segregate in steel. Since S forms elongated coarse MnS-based
inclusions so as to deteriorate hole expansion, the concentration
is preferably extremely low. In the conventional techniques, it was
necessary to significantly decrease the concentration of S in order
to secure hole expansion.
However, when an attempt is made to decrease the concentration of S
to less than 0.0001%, the desulfurization load during secondary
refining increases, and the desulfurization cost increases
excessively. In a case in which desulfurization during secondary
refining is assumed, when the desulfurization cost in accordance
with the quality of the steel sheet is taken into consideration,
the lower limit of the concentration of S is 0.0001%. Meanwhile, in
a case in which the costs for secondary refining are further
suppressed, and the effect of addition of Ce and La are more
effectively used, the concentration of S is preferably more than
0.0004%, more preferably 0.0005% or more, and most preferably
0.0010% or more.
In addition, in the present embodiment, MnS-based inclusions are
precipitated on fine and hard inclusions, such as Ce oxides, La
oxides, cerium oxysulfides, and lanthanum oxysulfides, so as to
control the morphology of MnS-based inclusions. Therefore,
inclusions do not easily deform during rolling, and elongation of
the inclusions is prevented. Therefore, the upper limit of the
concentration of S is specified by the relationship between the
concentration of S and the total amount of one or both of Ce and La
as described below. For example, the upper limit of the
concentration of S is 0.1%.
In the embodiment, since the morphology of MnS-based inclusions are
controlled by inclusions, such as Ce oxides, La oxides, cerium
oxysulfides, and lanthanum oxysulfides, even when the concentration
of S is high, it is possible to prevent S from adversely affecting
the qualities of the steel sheet by adding one or both of Ce and La
at an amount that corresponds to the concentration of S. That is,
even when the concentration of S increases to a certain extent, a
substantial desulfurization effect can be obtained by adding one or
both of Ce and La to steel at an amount that corresponds to the
concentration of S, and steel having the same qualities as
extremely low sulfur steel can be obtained.
In other words, since the concentration of S is appropriately
adjusted in accordance with the total amount of Ce and La, the
flexibility is large for the upper limit of the concentration of S.
As a result, in the embodiment, it is not necessary to carry out
desulfurization of the molten steel during the secondary refining
in order to obtain extremely low sulfur steel, and it is possible
to skip the secondary refining. Therefore, it is possible to
simplify the manufacturing processes of the steel sheet and,
accordingly, reduce the costs for the desulfurization.
Generally, since oxides of Al are liable to form clusters so as to
be coarse and deteriorate hole expansion, it is preferable to
suppress acid-soluble Al in the molten steel as much as possible.
However, the inventors newly found areas in which alumina-based
oxides are prevented from forming clusters so as to be coarse by
controlling the concentrations of Ce and La in the molten steel in
accordance with the concentration of the acid-soluble Al while Al
deoxidation is carried out. In the areas, of Al.sub.2O.sub.3-based
inclusions formed by the Al deoxidation, some of the
Al.sub.2O.sub.3-based inclusions are removed through floatation,
and the rest of the Al.sub.2O.sub.3-based inclusions in the molten
steel are reduced and decomposed by Ce and La that are to be added
afterwards, thereby forming fine inclusions.
Therefore, in the embodiment, it is substantially unnecessary to
add Al to steel, and, particularly, the flexibility is large for
the concentration of the acid-soluble Al. For example, the
concentration of the acid-soluble Al may be more than 0.004% in
consideration of the relationship between the concentration of the
acid-soluble Al and the total amount of one or both of Ce and La,
which will be described below.
In addition, in order to jointly use Al deoxidation and deoxidation
by the addition of Ce and La, the concentration of the acid-soluble
Al may be more than 0.010%. In this case, unlike the conventional
techniques, it becomes unnecessary to increase the amounts of Ce
and La in order to secure the total amount of deoxidizing elements,
the oxygen potential in steel can be further lowered, and variation
in the amount of each chemical element in the chemical composition
can be suppressed. Meanwhile, in a case in which the effect of
jointly using Al deoxidation and deoxidation by the addition of Ce
and La is further enhanced, the concentration of the acid-soluble
Al is preferably more than 0.020%, and more preferably more than
0.040%.
The upper limit of the concentration of the acid-soluble Al is
specified by the relationship between the acid-soluble Al and the
total amount of one or both of Ce and La as described below. For
example, the concentration of the acid-soluble Al may be 2.0% or
less in consideration of the above relationship.
Here, the concentration of the acid-soluble Al is determined by
measuring the concentration of Al that dissolves in an acid. For
analysis of the acid-soluble Al, the fact that dissolved Al (or
solute Al in a solid solution) dissolves in an acid, but
Al.sub.2O.sub.3 does not dissolve in an acid is used. Here,
examples of the acid include a mixed acid in which chloric acid,
nitric acid, and water are mixed at a ratio (mass ratio) of 1:1:2.
Using such an acid, Al that is soluble in the acid and
Al.sub.2O.sub.3 that is insoluble in the acid are separated, and
the concentration of the acid-soluble Al can be measured.
Meanwhile, the acid-insoluble Al (Al.sub.2O.sub.3 that is insoluble
in the acid) is determined as an inevitable impurity.
Ti is a major deoxidizing element, and increases the number of the
nucleation sites of austenite when carbides, nitrides, and
carbonitrides are formed, and the slabs are sufficiently heated
before hot rolling. As a result, since the grain growth of
austenite is suppressed, Ti contributes to refining of crystal
grains and an increase in the strength of the steel sheet, promotes
dynamic recrystallization during hot rolling, and significantly
improves hole expansion.
Therefore, in a case in which the above effect is sufficiently
enhanced, 0.008% or more of the acid-soluble Ti may be added to
steel. In a case in which the above effect does not need to be
sufficiently secured, and a case in which the slabs cannot be
sufficiently heated, the concentration of the acid-soluble Ti may
be less than 0.008%. Examples of imaginable situations in which the
slabs cannot be sufficiently heated include a case in which the
operation rate of the hot rolling is high and a case in which
sufficient heating capacity is not provided in the hot rolling.
Meanwhile, the lower limit of the concentration of the acid-soluble
Ti in steel is not particularly limited, but may be, for example,
0.0001% since Ti is inevitably included in steel.
In addition, when the concentration of the acid-soluble Ti exceeds
0.2%, the deoxidation effect of Ti is saturated, coarse carbides,
nitrides, and carbonitrides are formed by heating of the slabs
before hot rolling, and the qualities of the steel sheet
deteriorate. In this case, an effect in accordance with the
addition of Ti cannot be obtained. Therefore, in the embodiment,
the upper limit of the concentration of the acid-soluble Ti is
0.2%.
Therefore, the concentration of the acid-soluble Ti needs to be
0.0001% to 0.2%. In addition, in a case in which the effect of the
carbides, nitrides, and carbonitrides of Ti is sufficiently
secured, the concentration of the acid-soluble Ti is preferably
0.008% to 0.2%. In this case, in order to more reliably prevent the
carbides, nitrides, and carbonitrides of Ti from coarsening, the
concentration of the acid-soluble Ti may be 0.15% or less. On the
other hand, in a case in which the effect of the carbides,
nitrides, and carbonitrides of Ti and the deoxidation effect of Ti
are not sufficiently secured, the concentration of the acid-soluble
Ti is preferably more than or equal to 0.0001% and less than
0.008%.
When the slab is heated at a sufficient heating temperature before
hot rolling, carbides, nitrides, and carbonitrides formed during
casting can be made to temporarily dissolve so as to form solid
solutions. Therefore, in order to obtain an effect in accordance
with addition of Ti, the heating temperature before hot rolling is
preferably higher than 1200.degree. C. In this case, since fine
carbides, nitrides, and carbonitrides precipitates again from
solute Ti, it is possible to refine the crystal grains of the steel
sheet and increase the strength of the steel sheet. On the other
hand, the heating temperature before hot rolling exceeding
1250.degree. C. is not preferred from the viewpoint of costs and
scale forming. Therefore, the heating temperature before hot
rolling is preferably 1250.degree. C. or lower.
The concentration of the acid-soluble Ti is determined by measuring
the concentration of Ti dissolved in an acid. For analysis of the
acid-soluble Ti, the fact that dissolved Ti (or solute Ti in a
solid solution) dissolves in an acid, but Ti oxides do not dissolve
in an acid is used. Here, examples of the acid include a mixed acid
in which chloric acid, nitric acid, and water are mixed at a ratio
(mass ratio) of 1:1:2. Using such an acid, Ti that is soluble in
the acid and Ti oxides that are insoluble in the acid are
separated, and the concentration of the acid-soluble Ti can be
measured. Meanwhile, the acid-insoluble Ti (Ti oxides that are
insoluble in the acid) is determined as an inevitable impurity.
Ce and La are liable to reduce Al.sub.2O.sub.3 formed by Al
deoxidation and SiO.sub.2 formed by Si deoxidation, and serve as
precipitation sites of MnS-based inclusions. Furthermore, Ce and La
form inclusions (hard inclusions) including Ce oxides (for example,
Ce.sub.2O.sub.3 and CeO.sub.2), cerium oxysulfides (for example,
Ce.sub.2O.sub.2S), La oxides (for example, La.sub.2O.sub.3 and
LaO.sub.2), lanthanum oxysulfides (for example, La.sub.2O.sub.2S),
Ce oxide-La oxide, or cerium oxysulfide-lanthanum oxysulfide which
are hard and fine, and do not easily deform during rolling, as a
main compound (for example, the total amount of the compounds is
50% or more).
There are cases in which the hard inclusions include MnO,
SiO.sub.2, TiO.sub.2, Ti.sub.2O.sub.3, or Al.sub.2O.sub.3 due to
deoxidation conditions. However, when the main compound is the Ce
oxides, cerium oxysulfides, La oxides, lanthanum oxysulfides, Ce
oxide-La oxide, or cerium oxysulfide-lanthanum oxysulfide, the hard
inclusions sufficiently serve as the precipitation sites of
MnS-based inclusions while maintaining the size and hardness
thereof.
The inventors experimentally found that the total concentration of
one or both of Ce and La needs to be 0.001% to 0.04% in order to
obtain the above inclusions.
When the total concentration of one or both of Ce and La is less
than 0.001%, Al.sub.2O.sub.3 inclusions and SiO.sub.2 inclusions
cannot be reduced. In addition, when the total concentration of one
or both of Ce and La exceeds 0.04%, large amounts of cerium
oxysulfides and lanthanum oxysulfides are formed, and the
oxysulfides coarsen such that hole expansion deteriorate.
Therefore, the total of at least one selected from Ce and La is
preferably 0.001% to 0.04%. In order to more reliably reduce
Al.sub.2O.sub.3 inclusions and SiO.sub.2 inclusions, the total
concentration of one or both of Ce and La is most preferably 0.015%
or more.
In addition, the inventors paid attention to the fact that the
amount of MnS reformed by oxides or oxysulfides that includes one
or both of Ce and La (hereinafter sometimes also referred to as
"hard compounds") is expressed using the concentrations of Ce, La,
and S, and obtained an idea that the concentration of S and the
total concentration of Ce and La in steel are controlled using
([Ce]+[La])/[S].
Specifically, when ([Ce]+[La])/[S] is small, the amount of the hard
compounds is small, and a large amount of MnS alone precipitates.
When ([Ce]+[La])/[S] increases, the amount of the hard compounds
becomes larger than that of MnS, and inclusions having a morphology
in which MnS precipitates on the hard compounds increase. That is,
MnS is reformed by the hard compounds. As a result, hole expansion
are improved, and MnS is prevented from elongating.
That is, it is possible to use ([Ce]+[La])/[S] as a parameter that
controls the morphology of MnS-based inclusions. Therefore, the
inventors varied ([Ce]+[La])/[S] of the steel sheet, and evaluated
the morphology of inclusions and hole expansion in order to clarify
the composition ratio that is effective for suppressing the
elongation of MnS-based inclusions. As a result, it was found that,
when ([Ce]+[La])/[S] is 0.4 to 50, hole expansion are drastically
improved.
When ([Ce]+[La])/[S] is less than 0.4, the number percentage of
inclusions having a morphology in which MnS precipitates on the
hard compounds significantly decreases, and the number percentage
of MnS-based elongated inclusions that are liable to serve as
starting points of cracking increases such that hole expansion
degrade.
When ([Ce]+[La])/[S] exceeds 50, large amounts of formed cerium
oxysulfides and lanthanum oxysulfides form coarse inclusions, and
therefore hole expansion deteriorate. For example, when
([Ce]+[La])/[S] exceeds 70, cerium oxysulfides and lanthanum
sulfides form coarse inclusions having an equivalent circle
diameter of 50 .mu.m or more.
In addition, when ([Ce]+[La])/[S] exceeds 50, the effect of
controlling the morphology of MnS-based inclusions is saturated,
and thereby the effect which is appropriate for the costs cannot be
obtained. From the above results, ([Ce]+[La])/[5] needs to be 0.4
to 50. When the degree of controlling the morphology of MnS-based
inclusions and the costs are taken into account, ([Ce]+[La])/[S] is
preferably 0.7 to 30, and more preferably 1.0 to 10. Furthermore,
in a case in which the morphology of MnS-based inclusion are most
efficiently controlled while the chemical components in molten
steel is adjusted, ([Ce]+[La])/[S] is most preferably 1.1 or
more.
In addition, the inventors paid attention to the total
concentration of one or both of Ce and La with respect to the
concentration of the acid-soluble Al in the steel sheet of the
embodiment, which is obtained from molten steel that has undergone
deoxidation by Si, deoxidation by Al, and deoxidation by one or
both of Ce and La, and obtained an idea of using
([Ce]+[La])/[acid-soluble Al] as a parameter that appropriately
controls the oxygen potential in the molten steel.
The inventors experimentally found that, in a case in which
([Ce]+[La])/[acid-soluble Al] is 0.02 or more in the molten steel
that has undergone deoxidation by Si, deoxidation by Al, and then
deoxidation by at least one of Ce and La, it is possible to obtain
a steel sheet that is excellent in terms of hole expansion. In this
case, the oxygen potential in the molten steel abruptly decreases,
and, consequently, the concentration of Al.sub.2O.sub.3 formed
decreases. Therefore, even in a case in which deoxidation by Al is
actively carried out, similarly to a case in which deoxidation by
Al is rarely carried out, a steel sheet that is excellent in terms
of hole expansion could be obtained. In addition, in a case in
which ([Ce]+[La])/[acid-soluble Al] is less than 0.25, the costs
for Ce or La decreases, and transfer of oxygen between chemical
elements in the molten steel can also be efficiently controlled
based on the affinity of each chemical element to oxygen.
Meanwhile, in the embodiment, it is not necessary to actively carry
out deoxidation by Al, and simply necessary to control the total
concentration of at least one of Ca and La and the concentration of
the acid-soluble Al so that ([Ce]+[La])/[acid-soluble Al] satisfies
more than or equal to 0.02 and less than 0.25.
It was confirmed that, in a case in which ([Ce]+[La])/[acid-soluble
Al] is less than 0.02, the amount of Al added to at least one of Ca
and La becomes too large even when one or both of Ce and La are
added to steel, and therefore coarse alumina clusters that
deteriorate hole expansion are formed. In addition, in a case in
which ([Ce]+[La])/[acid-soluble Al] is 0.25 or more, there are
cases in which the morphology of inclusions are not sufficiently
controlled. For example, cerium oxysulfides and lanthanum
oxysulfides form coarse inclusions, and sufficient deoxidation is
not carried out in the molten steel. Therefore,
([Ce]+[La])/[acid-soluble Al] needs to be more than or equal to
0.02 and less than 0.25. In addition, in order to further reduce
the cost, and appropriately control the oxygen transfer between
chemical elements in the molten steel, ([Ce]+[La])/[acid-soluble
Al] is preferably less than 0.15, and more preferably less than
0.10. As such, even when desulfurization through the secondary
refining is not carried out, a steel sheet that is excellent in
terms of ductility and hole expansion can be obtained by
controlling ([Ce]+[La])/[S] and ([Ce]+[La])/[acid-soluble Al].
Hereinafter, in the embodiment, reasons why the amount of each
optional element in the chemical composition is limited will be
described. The chemical elements are optional elements, and can be
arbitrarily (optionally) added to steel. Therefore, the chemical
elements may not be added to steel, and at least one selected from
a group consisting of the chemical elements may be added to steel.
Meanwhile, since there are cases in which the chemical elements are
inevitably included in steel, the lower limit of the concentration
of the chemical elements is a threshold value that determines
inevitable impurities.
Nb, W, and V form carbides, nitrides, and carbonitrides with C or
N, promotes refining of the microstructure in a base metal, and
improves toughness.
In order to obtain complex carbides, complex nitrides, and the
like, 0.01% or more of Nb may be added to steel. However, even when
a large amount of Nb is added so that the concentration of Nb
exceeds 0.20%, the effect of refining the microstructure in the
base metal is saturated, and the manufacturing cost increases.
Therefore, the upper limit of the concentration of Nb is 0.20%. In
a case in which the cost of Nb is reduced, the concentration of Nb
may be controlled to 0.10% or less. Meanwhile, the lower limit of
the concentration of Nb is 0.001%.
In order to obtain the complex carbides, complex nitrides, and the
like, W may be added to steel. However, even when a large amount of
W is added so that the concentration of W exceeds 1.0%, the effect
of refining the microstructure in the base metal is saturated, and
the manufacturing cost increases. Therefore, the upper limit of the
concentration of W is 1.0%. Meanwhile, the lower limit of the
concentration of W is 0.001%.
In order to obtain complex carbides, complex nitrides, and the
like, 0.01% or more of V may be added to steel. However, even when
a large amount of V is added so that the concentration of V exceeds
1.0%, the effect of refining the microstructure in the base metal
is saturated, and the manufacturing cost increases. Therefore, the
upper limit of the concentration of V is 1.0%. In a case in which
the cost of V is reduced, the concentration of V may be controlled
to be 0.05% or less. Meanwhile, the lower limit of the
concentration of V is 0.001%.
Cr, Mo, and B are chemical elements that improve the hardenability
of steel.
Cr can be included in steel according to necessity in order to
further secure the strength of the steel sheet. For example, in
order to obtain the effect, 0.01% or more of Cr may be added to
steel. When a large amount of Cr is included in steel, the balance
between strength and ductility deteriorate. Therefore, the upper
limit of the concentration of Cr is 2.0%. In a case in which the
cost of Cr is reduced, the concentration of Cr may be controlled to
be 0.6% or less. In addition, the lower limit of the concentration
of Cr is 0.001%.
Mo can be included in steel according to necessity in order to
further secure the strength of the steel sheet. For example, in
order to obtain the effect, 0.01% or more of Mo may be added to
steel. When a large amount of Mo is included in steel, it becomes
difficult to suppress formation of pro-eutectic ferrite, and
therefore the balance between strength and ductility deteriorate.
Therefore, the upper limit of the concentration of Mo is 1.0%. In a
case in which the costs of Mo are reduced, the concentration of Mo
may be controlled to be 0.4% or less. In addition, the lower limit
of the concentration of Mo is 0.001%.
B can be included in steel according to necessity in order to
further strengthen grain boundaries and improve formability. For
example, in order to obtain the effect, 0.0003% or more of B may be
added to steel. Even when a large amount of B is included in steel,
the effect is saturated, the cleanliness of steel is impaired, and
the ductility deteriorates. Therefore, the upper limit of the
concentration of B is 0.005%. In a case in which the cost of B is
reduced, the concentration of B may be controlled to be 0.003% or
less. In addition, the lower limit of the concentration of B is
0.0001%.
In order to strengthen grain boundaries and improve formability by
controlling the morphology of sulfides, Ca, Mg, Zr, Sc, lanthanoids
of Pr through Lu (Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and
Yb) can be included in steel according to necessity.
Ca controls the morphology of sulfides by the spheroidizing of
sulfides or the like, so as to strengthen grain boundaries and
improve the formability of the steel sheet. For example, in order
to obtain the effect, the concentration of Ca may be 0.0001% or
more. Even when a large amount of Ca is included in steel, the
effect is saturated, the cleanliness of steel is impaired, and the
ductility deteriorates. Therefore, the upper limit of the
concentration of Ca is 0.01%. In a case in which the cost of Ca is
reduced, the concentration of Ca may be controlled to be 0.004% or
less. In addition, the lower limit of the concentration of Ca is
0.0001%.
Similarly, since Mg has almost the same effects as Ca, the
concentration of Mg is from 0.0001% to 0.01%.
In order to spheroidize sulfides so as to improve the toughness of
the base metal, 0.001% or more of Zr may be added to steel. When a
large amount of Zr is included in steel, the cleanliness of steel
is impaired, and the ductility deteriorates. Therefore, the upper
limit of the concentration of Zr is 0.2%. In a case in which the
cost of Zr is reduced, the concentration of Zr may be controlled to
be 0.01% or less. In addition, the lower limit of the concentration
of Zr is 0.0001%.
Similarly, in a case in which the morphology (shapes) of sulfides
is controlled, the total concentration of at least one selected
from Sc, and lanthanoids of Pr through Lu may be from 0.0001% to
0.1%.
In the embodiment, 0.001% to 2.0% of Cu and 0.001% to 2.0% of Ni
can be included in steel according to necessity. The chemical
elements improve hardenability so as to enhance the strength of
steel. Meanwhile, in a case in which quenching is efficiently
carried out using the chemical elements, the concentration of Cu
may be 0.04% to 2.0%, and the concentration of Ni may be 0.02% to
1.0%.
Furthermore, in a case in which scraps or the like are used as some
of starting materials, there are cases in which As, Co, Sn, Pb, Y,
and Hf are inevitably incorporated. In order to prevent the
chemical elements from adversely affecting the mechanical
properties (for example, hole expansion) of the steel sheet, the
concentration of each chemical elements is limited as below. The
upper limit of the concentration of As is 0.5%. The upper limit of
the concentration of Co is 1.0%. In addition, the upper limits of
the concentrations of Sn, Pb, Y, and Hf are all 0.2%. Meanwhile,
the lower limits of the chemical elements are all 0.0001%.
In the embodiment, the optional elements as described above can be
optionally included in steel.
Next, the microstructure of the high-strength steel sheet according
to the embodiment will be described.
Hole expansion are significantly affected by the local ductility of
a steel, and the most dominant factor in relation to hole expansion
is the difference in hardness between microstructures. Another
powerful dominant factor in relation to hole expansion is the
presence of nonmetallic inclusions, such as MnS. Generally, voids
are caused from the inclusions as the starting point, grow and link
together such that the steel breaks.
That is, when the hardness of the martensite phase is too large
compared to the hardness of other microstructures (for example, the
ferrite phase), there are cases in which, even when the morphology
of inclusions are controlled by adding Ce and La, and occurrence of
voids due to the inclusions is suppressed, stress concentrates at
the interfaces between ferrite and martensite, voids are caused due
to the difference in strength between the microstructures, and the
steel may break.
When the cooling conditions after hot rolling in the case of a
hot-rolled steel sheet, and the annealing conditions in the case of
a cold-rolled steel sheet are appropriately controlled, and the
hardness of the martensite phase is reduced, the effect of
suppressing occurrence of voids by controlling the morphology of
inclusions can be further enhanced. In this case, the effect of
controlling the morphology of inclusions by Ce and La that are
included in steel sheet is significantly exhibited as described
above. FIG. 1 schematically shows a relationship between the
maximum hardness (Vickers hardness) of martensite and hole
expansion ratios (hole expansion) .lamda.. As shown in FIG. 1, in a
case in which the hardness of the martensite phase is suppressed to
a certain value so that the morphology of inclusions are controlled
using at least one of Ce and La, hole expansion can be
significantly improved compared to a case in which the morphology
of inclusions are not controlled. In addition, in a microstructure
substantially composed of bainite, the degree of the hole expansion
improved by the addition of Ce and La is large, but the ductility
is poor compared to a steel sheet mainly including ferrite and
martensite.
In the embodiment, a steel sheet that is excellent in terms of both
hole expansion and ductility is provided. Therefore, the major
microstructure is ferrite and martensite, and the microstructure
includes 1% to 50% of the martensite phase in terms of the area
ratio, optionally includes bainite and retained austenite, and has
a remainder composed of a ferrite phase. In this case, in order to
obtain uniform deformability, for example, bainite and retained
austenite are controlled to 10% or less each. When the area ratio
of the martensite phase is less than 1%, the work-hardenability is
weak. In order to further enhance the work-hardenability, the area
ratio of the martensite phase is preferably 3% or more, and more
preferably 5% or more. On the other hand, when the area ratio of
the martensite phase exceeds 50%, the uniform deformability of the
steel sheet decreases significantly. In order to obtain a large
uniform deformability, the area ratio of the martensite phase is
preferably 30% or less, and more preferably 20% or less. Meanwhile,
some or all of the martensite phase may be tempered martensite. The
ratio of the martensite phase is determined by the area ratio of
the martensite phase in a microstructure photograph obtained using
an optical microscope. Herein, the inclusions as described below
are included in the microstructures (the martensite phase, the
ferrite phase, the bainite, and the retained austenite).
Since the hardness of the ferrite phase and the martensite phase
included in steel varies with the chemical composition and
manufacturing conditions (for example, the amount of strains caused
during rolling or cooling rate) of steel, the hardness is not
particularly limited. When it is taken into account that the
hardness of the martensite phase is high compared to those of other
microstructures, the maximum hardness of the martensite phase
included in steel is preferably 600 Hv or less. The maximum
hardness of the martensite phase is the maximum value of
micro-Vickers hardness obtained by randomly pressing an indenter
with a load of 10 gf on a hard phase (other than the ferrite phase)
50 times.
Next, the presence conditions of inclusions in the high-strength
steel sheet of the embodiment will be described. Here, the steel
sheet refers to a rolled sheet obtained after hot rolling or cold
rolling.
In the embodiment, the presence conditions of inclusions in the
steel sheet can be optionally specified from a variety of
viewpoints.
In the first feature in relation to inclusions, the number density
of inclusions that are present in the steel sheet and have an
equivalent circle diameter of 0.5 .mu.m to 2 .mu.m is 15
inclusions/mm.sup.2 or more.
In order to obtain a steel sheet that is excellent in terms of
ductility and hole expansion, it is important to reduce as much as
possible elongated coarse MnS-based inclusions that easily act as
starting points of cracking or crack propagation paths.
As described above, the inventors found that, in a case in which
([Ce]+[La])/[acid-soluble Al] and ([Ce]+[La])/[S] are in the above
ranges, since the oxygen potential in the molten steel abruptly
lowers due to the complex deoxidations, and the concentration of
Al.sub.2O.sub.3 in inclusions decreases, a steel sheet that is
deoxidized by Si, then, deoxidized by Al, and then deoxidized by at
least one of Ce and La is excellent in terms of ductility and hole
expansion, similarly to a steel sheet manufactured with little
deoxidation by Al.
In addition, the inventor also found that, since MnS precipitates
on fine and hard Ce oxides, La oxides, cerium oxysulfides, and
lanthanum oxysulfides that are formed due to deoxidation by the
addition of Ce and La, and the precipitated MnS does not easily
deform during rolling, the elongated coarse MnS is significantly
reduced in the steel sheet.
That is, it was found that, in a case in which
([Ce]+[La])/[acid-soluble Al] and ([Ce]+[La])/[S] are in the above
ranges, the number density of fine inclusions having an equivalent
circle diameter of 2 .mu.M or less abruptly increases, and the fine
inclusions are dispersed in steel.
Since the fine inclusions do not easily aggregate, most of the
inclusions have a spherical or spindle shape. In addition, since
inclusions having MnS precipitated on Ce oxides, La oxides, cerium
oxysulfides, and lanthanum oxysulfides have a high melting point
and do not easily deform, the inclusions maintain an almost
spherical shape even during hot rolling. As a result, the long
diameter/short diameter (hereinafter sometimes referred to as the
"elongation ratio") of most of the inclusions is generally 3 or
less.
Since the likelihood of the inclusions to serve as starting points
of fracture significantly varies with the shapes of the inclusions,
the elongation ratio of the inclusions is preferably 2 or less.
Experimentally, attention was paid to the number density of
inclusions having an equivalent circle diameter of 0.5 .mu.m to 2
.mu.m so that the inclusions can be easily identified through
observation using a scanning electron microscope (SEM) or the like.
With regard to the lower limit of the equivalent circle diameter,
inclusions that are large enough to be sufficiently counted are
used. That is, the number of inclusions was counted with respect to
inclusions of 0.5 .mu.m or more. The equivalent circle diameter is
obtained by measuring the long diameter and the short diameter of
an inclusion observed on a cross-section, and computing (long
diameter.times.short diameter).sup.0.5.
While the detailed mechanism is not clear, it is considered that
fine inclusions of 2 .mu.m or less are dispersed in the
microstructure at 15 inclusions/mm.sup.2 or more due to a
synergetic effect of the lowering of the oxygen potential in the
molten steel by Al deoxidation and the refining of MnS-based
inclusions. It is assumed that, due to the above, stress
concentration caused during forming of hole expanding or the like
is alleviated, and an effect of abruptly improving hole expansion
is exhibited. As a result, it is considered that, during repetitive
deformation or hole expanding, MnS-based inclusions are fine, and
therefore MnS-based inclusions do not easily act as starting points
of cracking or crack propagation paths, alleviate stress
concentration, and improve formability, such as hole expansion. As
such, with regard to the morphology of the inclusions, the number
density of inclusions that are present in the steel sheet and have
an equivalent circle diameter of 0.5 .mu.m to 2 .mu.m is preferably
15 inclusions/mm.sup.2 or more.
In the second feature in relation to inclusions, of inclusions that
are present in the steel sheet and have an equivalent circle
diameter of 1 .mu.m or more, the number percentage of elongated
inclusions having an aspect ratio (elongation ratio) of 5 or more
obtained by dividing the long diameter by the short diameter is 20%
or less.
The inventors investigated whether or not elongated coarse
MnS-based inclusions that easily act as starting points of cracking
or crack propagation paths are reduced.
The inventor experimentally found that, when the equivalent
circular diameters of the inclusions are less than 1 .mu.m, even in
a case in which MnS is elongated, the inclusions do not act as
starting points of cracking, and ductility and hole expansion are
not deteriorated. In addition, since inclusions having an
equivalent circle diameter of 1 .mu.m or more can be easily
observed using a scanning electron microscope (SEM) or the like,
the morphology and chemical compositions of inclusions having an
equivalent circle diameter of 1 .mu.m or more in the steel sheet
were investigated, and the distribution of the elongated MnS was
evaluated. The upper limit of the equivalent circle diameter of MnS
is not particularly specified; however, for example, there are
cases in which MnS of approximately 1 mm is observed in the steel
sheet.
The number percentage of elongated inclusions is obtained in the
following manner. Here, the elongated inclusion refers to an
inclusion having a long diameter/short diameter (elongation ratio)
of 5 or more.
The chemical compositions of a plurality (for example, a
predetermined number of 50 or more) of inclusions having an
equivalent circle diameter of 1 .mu.m or more which are randomly
selected using a SEM are analyzed, and the long diameter and short
diameter of the inclusions are measured from a SEM image (secondary
electron image). The number percentage of the elongated inclusions
can be obtained by dividing the number of the detected elongated
inclusions by the number of all inclusions investigated (in the
above example, a predetermined number of 50 or more).
A reason why the elongated inclusions are defined as inclusions
having an elongation ratio of 5 or more is that most of inclusions
having an elongation ratio of 5 or more in the steel sheet to which
Ce and La are not added are MnS. The upper limit of the elongation
ratio of MnS is not particularly specified; however, for example,
there are cases in which MnS having an elongation ratio of
approximately 50 is observed in the steel sheet.
As a result of evaluation by the inventors, it was found that, in
the steel sheets for which the number percentage of the elongated
inclusions having an elongation ratio of 5 or more with respect to
inclusions having an equivalent circle diameter of 1 .mu.m or more
is controlled to be 20% or less, the hole expansion are improved.
When the number percentage of the elongated inclusions exceeds 20%,
since a number of MnS-based elongated inclusions that easily act as
starting points of cracking are present, the hole expansion
degrades. In addition, as the grain sizes of the elongated
inclusions increase, that is, as the equivalent circle diameters
increase, stress concentration occurs more easily during forming
and deformation, and therefore the elongated inclusions easily act
as starting points of cracking or crack propagation paths, and the
hole expansion abruptly deteriorates.
Therefore, in the embodiment, the number percentage of the
elongated inclusions is preferably 20% or less. Since the hole
expansion become better as the elongated MnS-based inclusions
become smaller, the lower limit of the number percentage of the
elongated inclusions include 0%.
In a case in which inclusions having an equivalent circle diameter
of 1 .mu.m or more are included, and elongated inclusions having an
elongation ratio of 5 or more are not present in the inclusions, or
in a case in which the equivalent circle diameters of inclusions
are all less than 1 .mu.m, the number percentage of elongated
inclusions having an elongation ratio of 5 or more in inclusions
having an equivalent circle diameter of 1 .mu.m or more is
determined to be 0%.
It is confirmed that the maximum equivalent circle diameters of
elongated inclusions are also small compared to the average grain
size of crystals (metallic crystals) in the microstructure, and the
reduction of the maximum equivalent circle diameters of the
elongated inclusions are also considered to be a factor that can
drastically improve the hole expansion.
In the third feature in relation to inclusions, of inclusions
having an equivalent circle diameter of 1.0 .mu.m or more in the
steel sheet, the number percentage of inclusions having at least
one of MnS, TiS, and (Mn, Ti)S precipitated on an oxide or
oxysulfide composed of at least one of Ce and La, and at least one
of O and S, or an oxide or oxysulfide composed of at least one of
Ce and La, at least one of Si and Ti, and at least one of O and S
is 10% or more.
For example, in a steel sheet having ([Ce]+[La])/[S] of 0.4 to 50,
MnS-based inclusions precipitate on an oxide or oxysulfide
including one or both of Ce and La, or an oxide or oxysulfide
including one or both of Ce and La, and one or both of Si and Ti
(the above hard compounds). Meanwhile, in a steel sheet in which
the acid-soluble Ti is less than 0.008%, there are many cases in
which oxides or oxysulfides including one or both of Si and Ti are
not formed.
The morphology of the inclusions is not particularly specified as
long as MnS-based inclusions precipitate on the hard compounds, and
there are many cases in which MnS-based inclusions precipitate
around the hard compounds as nuclei.
Also, there are cases in which TiN precipitates together with
MnS-based inclusions on the fine and hard Ce oxides, La oxides,
cerium oxysulfides, and lanthanum oxysulfides. However, since TiN
has little influence on ductility and hole expansion as described
above, TiN itself is not included in MnS-based inclusions.
Since inclusions having MnS-based inclusions precipitated on the
hard compounds in the steel sheet do not easily deform during
rolling, the inclusions have a shape that is not elongated, that
is, a spherical or spindle shape.
Here, inclusions that are determined to be not elongated (spherical
inclusions) are not particularly specified; however, for example,
the inclusions are an inclusion having an elongation ratio of 3 or
less, and preferably an inclusion having an elongation ratio of 2
or less. This is because the elongation ratio of an inclusion
having MnS-based inclusions precipitated on the hard compounds in a
slab before rolling is 3 or less. In addition, when the spherical
inclusion is a perfectly spherical body, the elongation ratio is 1,
and therefore the lower limit of the elongation ratio is 1.
The inventors investigated the number percentage of the inclusions
(spherical inclusions) by the same method as the method of
measuring the number percentage of the elongated inclusions. That
is, the chemical compositions of a plurality (for example, a
predetermined number of 50 or more) of inclusions having an
equivalent circle diameter of 1.0 .mu.m or more which are randomly
selected using a SEM are analyzed, and the long diameter and short
diameter of the inclusions are measured from a SEM image (secondary
electron image). The number percentage of the spherical inclusions
can be obtained by dividing the number of the spherical inclusions
having a detected elongation ratio of 3 or less by the number of
all inclusions investigated (in the above example, a predetermined
number of 50 or more). As a result, in the steel sheet for which
the number percentage of inclusions having MnS-based inclusions
precipitated on the hard compounds (spherical inclusions) is
controlled to be 10% or more, the hole expansion are improved.
When the number percentage of the inclusions having MnS-based
inclusions precipitated on the hard compounds is less than 10%, the
number percentage of MnS-based elongated inclusions increases, and
the hole expansion degrades. Therefore, in the embodiment, of the
inclusions having an equivalent circle diameter of 1.0 .mu.m or
more, the number percentage of inclusions having MnS-based
inclusions precipitated on the hard compounds is 10% or more.
Since the hole expansion are improved by precipitating a number of
MnS-based inclusion on the hard compounds, the upper limit value of
the number percentage of inclusions having MnS-based inclusions
precipitated on the hard compounds includes 100%.
Meanwhile, since the inclusions having MnS-based inclusions
precipitated on the hard compounds do not easily deform during
rolling, the equivalent circle diameter is not particularly
specified, and hole expansion are not adversely affected even when
the equivalent circle diameter is 1 .mu.m or more. However, when
the equivalent circle diameter is too large, there is a possibility
for inclusions to act as starting points of cracking, and therefore
the upper limit of the equivalent circle diameter is preferably
approximately 50 .mu.m.
Additionally, in a case in which the equivalent circle diameters of
inclusions are less than 1 .mu.m, since the inclusions do not
easily act as starting points of cracking, the lower limit of the
equivalent circle diameter is not specified.
In the fourth feature in relation to inclusions, of inclusions that
are present in the steel sheet and have an equivalent circle
diameter of 1 .mu.m or more, the volume number density of elongated
inclusions having an aspect ratio of 5 or more obtained by dividing
the long diameter by the short diameter (elongation ratio) is
1.0.times.10.sup.4 inclusions/mm.sup.3 or less.
The grain size distribution of inclusions is obtained through, for
example, SEM observation of electrolyzed surfaces according to the
SPEED method (Selective Potentiostatic Etching by Electrolytic
Dissolution method). In the SEM observation of an electrolyzed
surface by the SPEED method, a surface of a test specimen obtained
from a steel sheet is polished, then, electrolyzed by the SPEED
method, and the sample surface is directly observed using a SEM,
whereby the sizes and number density of inclusions are
evaluated.
The SPEED method is a method in which a metal matrix on the sample
surface is electrolyzed using a solution of 10% acetyl acetone, 1%
tetramethyl ammonium chloride, and methanol, and inclusions are
shown. The electrolysis is performed, for example, in 1 coulomb per
an area of the sample surface of 1 cm.sup.2. A SEM image on the
electrolyzed sample surface is processed by an image-processing,
and the equivalent circle diameter and frequency (number)
distribution of inclusions are obtained. The frequency distribution
is divided by the depth of electrolysis so as to compute the number
density of inclusions per volume.
The inventors evaluated the volume number density of elongated
inclusions having an equivalent circle diameter of 1 .mu.m or more
and an elongation ratio of 5 or more as inclusions that act as
starting points of cracking and deteriorate hole expansion. As a
result, it was found that, when the volume number density of the
elongated inclusion is 1.0.times.10.sup.4 inclusions/mm.sup.3 or
less, hole expansion improves.
When the volume number density of the elongated inclusions exceeds
1.0.times.10.sup.4 inclusions/mm.sup.3, the number density of
MnS-based elongated inclusions that easily act as starting points
of cracking increases, and hole expansion degrade. Therefore, the
volume number density of elongated inclusions having an equivalent
circle diameter of 1 .mu.m or more and an elongation ratio of 5 or
more is limited to 1.0.times.10.sup.4 inclusions/mm.sup.3 or less.
Since hole expansion improve as elongated MnS-based inclusions
decrease, the lower limit value of the volume number density of the
elongated inclusions includes 0%.
Meanwhile, similarly to the second feature in relation to
inclusions, it is found that, in a case in which inclusions having
an equivalent circle diameter of 1 .mu.m or more and an elongation
ratio of 5 or more are not present, or a case in which the
equivalent circle diameters of inclusions are all less than 1
.mu.m, of inclusions having an equivalent circle diameter of 1
.mu.m or more, the volume number density of elongated inclusion
having an elongation ratio of 5 or more is 0%.
In the fifth feature in relation to inclusions, of inclusions
having an equivalent circle diameter of 1 .mu.m or more in the
steel sheet, the volume number density of inclusions having at
least one of MnS, TiS, and (Mn, Ti)S precipitated on an oxide or
oxysulfide (hard compound) composed of at least one of Ce and La,
and at least one of O and S, or an oxide or oxysulfide composed of
at least one of Ce and La, at least one of Si and Ti, and at least
one of O and S is 1.0.times.10.sup.3 inclusions/mm.sup.3 or
more.
Investigation by the inventors showed that unelongated MnS-based
inclusions had MnS-based inclusions precipitated on the hard
compounds and had an almost spherical or spindle shape.
The morphology of the inclusions are not particularly specified as
long as MnS-based inclusions are precipitated on the hard
compounds, but there are many cases in which MnS-based inclusions
precipitate around the hard compounds as nuclei.
The spherical inclusion is defined in the same manner as in the
third feature in relation to inclusions, and, the volume number
density of the spherical inclusions is measured using the same
SPEED method as in the fourth feature in relation to
inclusions.
As a result of investigation by the inventor on the volume number
density of the spherical inclusions, it was found that in steel
sheets for which the volume number density of inclusions having
MnS-based inclusions precipitated around the hard compounds as
nuclei (spherical inclusions) is controlled to be
1.0.times.10.sup.3 inclusions/mm.sup.3 or more, hole expansion
improves.
When the volume number density of inclusions having MnS-based
inclusions precipitated on the hard compounds becomes less than
1.0.times.10.sup.3 inclusions/mm.sup.3, the number percentage of
MnS-based elongated inclusions increases, and hole expansion
degrades. Therefore, the volume number density of inclusion having
MnS-based inclusion precipitated on the hard compounds is
1.0.times.10.sup.3 inclusions/mm.sup.3 or more. Since hole
expansion are improved by precipitating a number of MnS-based
inclusions using the hard compounds as nuclei, the upper limit of
the volume number density is not specified.
The equivalent circle diameters of inclusions having MnS-based
inclusions precipitated on the hard compounds are not particularly
specified. However, when the equivalent circle diameter is too
large, there is a possibility for inclusions to act as starting
points of cracking, and therefore the upper limit of the equivalent
circle diameter is preferably approximately 50 .mu.m.
Additionally, in a case in which the equivalent circle diameters of
inclusions are less than 1 .mu.m, no problem occurs, and therefore
the lower limit of the equivalent circle diameter is not
specified.
In the sixth feature in relation to inclusions, of inclusions that
are present in the steel sheet and have an equivalent circle
diameter of 1 .mu.m or more, the average equivalent circle diameter
of inclusions having an aspect ratio of 5 or more obtained by
dividing the long diameter by the short diameter (elongation ratio)
is 10 .mu.m or less.
The inventors evaluated the average equivalent circle diameter of
elongated inclusions having an equivalent circle diameter of 1
.mu.m or more and a elongation ratio of 5 or more as inclusions
that act as starting points of cracking and deteriorate hole
expansion. As a result, it was found that, when the average
equivalent circle diameter of the elongated inclusions is 10 .mu.m
or less, hole expansion improves. This is assumed to be because, as
the amount of Mn or S in the molten steel increases, the number of
MnS-based inclusions being formed increases, and the sizes of
MnS-based inclusions being formed also increase.
As a result, attention was paid to a phenomenon in which the
average equivalent circle diameter of the elongated inclusions
increases as the number percentage of the elongated inclusions
increases, and the average equivalent circle diameter of the
elongated inclusions was specified as a parameter.
When the average equivalent circle diameter of the elongated
inclusions exceeds 10 .mu.m, the number percentage of coarse
MnS-based inclusions that easily act as starting points of cracking
increases. As a result, hole expansion degrades, and therefore the
morphology of inclusions is controlled so that the average
equivalent circle diameter of the elongated inclusions having
equivalent circle diameter of 1 .mu.m or more and an elongation
ratio of 5 or more becomes 10 .mu.m or less.
Since the average equivalent circle diameter of the elongated
inclusions is obtained by measuring the equivalent circle diameters
of inclusions that are present in the steel sheet and have an
equivalent circle diameter of 1 .mu.m or more using a SEM, and
dividing the total of equivalent circle diameters of a plurality
(for example, a predetermined number of 50 or more) of inclusions
by the number of the plurality of inclusions, the lower limit of
the average equivalent circle diameter is 1 .mu.m.
In the seventh feature in relation to inclusion, inclusions having
at least one of MnS, TiS, and (Mn, Ti)S precipitated on an oxide or
oxysulfide composed of at least one of Ce and La, and at least one
of O and S, or an oxide or oxysulfide composed of at least one of
Ce and La, at least one of Si and Ti, and at least one of O and S
are present in the steel sheet, and the inclusions include a total
of 0.5 mass % to 95 mass % of at least one of Ce and La in terms of
an average chemical composition.
As described above, in order to improve hole expansion, it is
important to precipitate MnS-based inclusions on the hard compounds
and prevent elongation of MnS-based inclusions. With regard to the
morphology of the inclusions, MnS-based inclusions may be
precipitated on hard inclusions, and, generally, MnS-based
inclusions precipitate around hard inclusions as nuclei.
The inventors analyzed the chemical compositions of inclusions
having MnS-based inclusions precipitated on the hard inclusions
through SEM and energy dispersive X-ray spectroscopy (EDX) in order
to clarify the chemical compositions of inclusions, which are
effective for suppressing elongation of MnS-based inclusions. When
the equivalent circle diameters of the inclusions are 1 .mu.m or
more, since inclusions are easily observed, the composition
analysis was carried out on inclusions having an equivalent circle
diameter of 1 .mu.m or more. In addition, since inclusions having
MnS-based inclusions precipitated on hard inclusions are not
elongated as described above, the elongation ratios are all 3 or
less. Therefore, the composition analysis was carried out on
spherical inclusions having an equivalent circle diameter of 1
.mu.m or more and an elongation ratio of 3 or less, which are
defined in the third feature in terms of inclusions.
As a result, it was found that, when the spherical inclusions
include a total of 0.5% to 95% of one or both of Ce and La in terms
of an average chemical composition, hole expansion improves.
When the average amount of the sum of one or both of Ce and La in
the spherical inclusions is less than 0.5 mass %, the number
percentage of inclusions having MnS-based inclusions precipitated
on the hard compounds significantly decreases, and therefore the
number percentage of MnS-based elongated inclusions that easily act
as starting points of cracking increases, and hole expansion and
fatigue characteristics degrade. Meanwhile, the larger the average
amount of the sum of one or both of Ce and La, the more preferable.
For example, the upper limit of the average amount may be 95% or
50% according to the amount of MnS-based inclusions.
When the average amount of the sum of one or both of Ce and La in
the spherical inclusions exceeds 95%, large amounts of cerium
oxysulfides and lanthanum oxysulfides form coarse inclusions having
an equivalent circle diameter of 50 .mu.m or more, hole expansion
and fatigue characteristics deteriorate.
Meanwhile, the high-strength steel sheet of the embodiment may be a
cold-rolled steel sheet or a hot-rolled steel sheet. In addition,
the high-strength steel sheet of the embodiment may be a coated
steel sheet having a coating, such as a galvanized layer or a
galvannealed layer, on at least one surface thereof.
Next, the manufacturing conditions of the high-strength steel sheet
according to an embodiment of the present invention will be
described. Meanwhile, the chemical composition of the molten steel
is the same as the chemical composition of the high-strength steel
sheet of the above embodiment.
In the present invention, an alloy of C, Si, Mn, and the like is
added to molten steel that has been blown and decarburized in a
converter, and stirred so as to carry out deoxidization and adjust
the chemical components. Meanwhile, according to necessity,
deoxidization can be carried out using a vacuum degassing
apparatus.
Meanwhile, with regard to S, since desulfurization need not be
carried out in the refining process as described above, a
desulfurization process can be skipped. However, in a case in which
desulfurization of the molten steel is required in secondary
refining in order to melt extremely low sulfur steel having a
concentration of S of 20 ppm or less, the amount of the chemical
components may be controlled by carrying out desulfurization.
Deoxidation and composition control are carried out in the
following manner.
After Si (for example, Si or a compound including Si) is added to
the molten steel, and approximately three minutes pass, Al (for
example, Al or a compound including Al) is added to the molten
steel, and deoxidization is carried out. A floatation time of
approximately 3 minutes is preferably secured in order to make
oxygen and Al combine together so as to float Al.sub.2O.sub.3.
After that, in a case in which addition of Ti (for example, Ti or a
compound including Ti) is required, Ti is added to the molten
steel. In this case, a floatation time of approximately 2 to 3
minutes is preferably secured in order to make oxygen and Ti
combine together so as to float TiO.sub.2 and Ti.sub.2O.sub.3.
After that, the chemical composition are controlled by adding one
or both of Ce and La to the molten steel so as to satisfy
0.02.ltoreq.([Ce]+[La])/[acid-soluble Al]<0.25, and
0.4.ltoreq.([Ce]+[La])/[S].ltoreq.50.
In a case in which optional elements are added, addition of the
optional elements is completed before one or both of Ce and La are
added to the molten steel. In this case, the molten steel is
sufficiently stirred so as to adjust the amounts of the optional
elements, and then one or both of Ce and La are added to the molten
steel. The molten steel manufactured in the above manner is
subjected to continuous casting so as to manufacture slabs.
With regard to the continuous casting, the embodiment can be
sufficiently applied not only to ordinary slab continuous casting
in which approximately 250 mm-thick slabs are manufactured but also
to, for example, thin slab continuous casting in which 150 mm or
less-thick slabs are manufactured.
In the embodiment, the high-strength hot-rolled steel sheet can be
manufactured in the following manner.
The obtained slab is reheated to 1100.degree. C. or higher, and
preferably 1150.degree. C. or higher according to necessity.
Particularly, in a case in which it is necessary to sufficiently
control the morphology (for example, fine precipitation) of
carbides and nitrides, it is necessary to temporarily form solid
solutions by dissolving carbides and nitrides in steel, and
therefore the heating temperature of the slab before hot rolling
preferably exceeds 1200.degree. C. A ferrite phase whose ductility
is improved in a cooling process after rolling can be obtained by
forming solid solutions by dissolving carbides and nitrides in
steel.
When the heating temperature of the slab before hot rolling exceeds
1250.degree. C., there are cases in which the surfaces of the slab
are significantly oxidized. Particularly, there are cases in which
wedge-shaped surface defects caused by selective oxidation of grain
boundaries are liable to remain after descaling, and the qualities
of the surfaces after rolling are impaired. Therefore, the upper
limit of the heating temperature is preferably 1250.degree. C.
Meanwhile, the heating temperature is preferably as low as possible
in terms of costs.
Next, hot rolling is carried out at a finishing temperature of
850.degree. C. to 970.degree. C. on the slab so as to manufacture a
steel sheet. When the finishing temperature is lower than
850.degree. C., the rolling is carried out in a two-phase region,
and therefore ductility degrades. When the finishing temperature
exceeds 970.degree. C., austenite grain sizes become coarse, the
ratio of the ferrite phase decreases, and ductility degrades.
After the hot rolling, the steel sheet is cooled to a temperature
range of 450.degree. C. or lower (cooling control temperature) at
an average cooling rate of 10.degree. C./second to 100.degree.
C./second, the steel sheet is coiled in a temperature of
300.degree. C. to 450.degree. C. (coiling temperature). A
hot-rolled steel sheet is manufactured as a final product in the
above manner. In a case in which the cooling control temperature
after hot rolling is higher than 450.degree. C., a ratio of desired
martensite phase cannot be obtained, and therefore the upper limit
of the coiling temperature is 450.degree. C. Meanwhile, in a case
in which the martensite phase is secured more flexibly, the upper
limits of the cooling control temperature and the coiling
temperature are preferably 440.degree. C. When the coiling
temperature is 300.degree. C. or lower, the hardness of the
martensite phase excessively increases, and therefore the lower
limit of the coiling temperature is 300.degree. C.
In addition, when the cooling rate is less than 10.degree.
C./second, pearlite is liable to be formed, and, when the cooling
rate exceeds 100.degree. C./second, it is difficult to control the
coiling temperature.
When a hot-rolled steel sheet is manufactured by controlling the
hot rolling conditions and the cooling conditions after hot rolling
in the above manner, a high-strength steel sheet that is excellent
in terms of hole expansion and ductility, and mainly includes
ferrite and martensite can be manufactured.
In addition, in the embodiment, the high-strength cold-rolled steel
sheet can be manufactured in the following manner.
After the casting, the slab having the above chemical composition
is reheated to 1100.degree. C. or higher according to necessity.
Meanwhile, reasons why the temperature of the slab before the hot
rolling is controlled are the same as in a case in which the above
high-strength hot-rolled steel sheet is manufactured.
Next, hot rolling is carried out at a finishing temperature of
850.degree. C. to 970.degree. C. on the slab so as to manufacture a
steel sheet. Furthermore, the steel sheet is cooled to a
temperature range of 300.degree. C. to 650.degree. C. (cooling
control temperature) at an average cooling rate of 10.degree.
C./second to 100.degree. C./second. After that, the steel sheet is
coiled at a temperature of 300.degree. C. to 650.degree. C.
(coiling temperature) so as to manufacture a hot-rolled steel sheet
as an intermediate material.
When the cooling control temperature and the coiling temperature
are higher than 650.degree. C., lamellar pearlite is liable to be
formed, and the lamellar pearlite cannot be sufficiently melt
through annealing, and therefore hole expansion degrades. In
addition, when the coiling temperature is lower than 300.degree.
C., the hardness of the martensite phase excessively increases, and
therefore it is difficult to efficiently coil the steel sheet.
Meanwhile, reasons why the cooling rate and the finishing
temperature of the hot rolling are limited are the same as in a
case in which the above high-strength hot-rolled steel sheet is
manufactured.
The hot-rolled steel sheet (steel sheet) manufactured in the above
manner is pickled, then, subjected to cold rolling at a reduction
in thickness of 40% or more, and annealed at a maximum temperature
of 750.degree. C. to 900.degree. C. After that, the steel sheet is
cooled to 450.degree. C. or lower at an average cooling rate of
0.1.degree. C./second to 200.degree. C./second, and, subsequently,
held for 1 second to 1000 seconds in a temperature range of
300.degree. C. to 450.degree. C. A high-strength cold-rolled steel
sheet that is excellent in terms of elongation and hole expansion
can be manufactured as a final product in the above manner.
In manufacturing the cold-rolled steel sheet, when the reduction in
thickness is less than 40%, it is not possible to sufficiently
refine crystal grains after the annealing.
In a case in which the maximum temperature of the annealing is
lower than 750.degree. C., the amount of austenite obtained through
the annealing is small, and therefore it is not possible to form a
desired amount of martensite in the steel sheet. When the annealing
temperature increases, the grain sizes of the austenite becomes
coarse, ductility degrades, and manufacturing cost increases, and
therefore the upper limit of the maximum temperature of the
annealing is 900.degree. C.
The cooling after the annealing is important to promote
transformation from austenite to ferrite and martensite. When the
cooling rate is less than 0.1.degree. C./second, since pearlite is
formed such that hole expansion and strength degrade, the lower
limit of the cooling rate is 0.1.degree. C./second. In a case in
which the cooling rate exceeds 200.degree. C./second, it is not
possible to sufficiently proceed with ferrite transformation, and
ductility degrades, and therefore the upper limit of the cooling
rate is 200.degree. C./second.
The cooling temperature during the cooling after the annealing is
450.degree. C. or lower. When the cooling temperature exceeds
450.degree. C., it is difficult to form martensite. Next, the
cooled steel sheet is held in a temperature range of 300.degree. C.
to 450.degree. C. for 1 second to 1000 seconds.
A reason why the lower limit of the cooling temperature cannot be
provided is that martensite transformation can be promoted by once
cooling the steel sheet to a temperature lower than the holding
temperature. Meanwhile, even when the cooling temperature is
300.degree. C. or lower, as long as the steel sheet is held in a
temperature higher than the cooling temperature, the martensite is
tempered, and it is possible to reduce the difference in hardness
between the martensite and the ferrite.
When the holding temperature is lower than 300.degree. C., the
hardness of the martensite phase excessively increases. In
addition, when the holding time is less than 1 second, thermal
shrinkage-induced residual strains remain, and elongation degrades.
When the holding time exceeds 1000 seconds, more bainite and the
like are formed than is necessary, and a desired amount of
martensite cannot be formed.
As described above, when a hot-rolled steel sheet is manufactured
by controlling the hot rolling conditions and the cooling
conditions after the hot rolling, and a cold-rolled steel sheet is
manufactured from the hot-rolled steel sheet by controlling the
cold rolling conditions, the annealing conditions, the cooling
conditions, and the holding conditions, it is possible to
manufacture a high-strength cold-rolled steel sheet that is
excellent in terms of hole expansion and ductility, and mainly
includes ferrite and martensite.
Therefore, in the embodiment, molten steel is processed into a
slab, hot rolling is carried out on the slab at a finishing
temperature of 850.degree. C. to 970.degree. C. so as to
manufacture a steel sheet. After that, the steel sheet is cooled to
a cooling control temperature of 650.degree. C. or lower at an
average cooling rate of 10.degree. C./second to 100.degree.
C./second, and then coiled at a coiling temperature of 300.degree.
C. to 650.degree. C. Here, in a case in which a hot-rolled steel
sheet is manufactured, the cooling control temperature is
450.degree. C. or lower, and the coiling temperature is 300.degree.
C. to 450.degree. C. In addition, when a cold-rolled steel sheet is
manufactured, the coiled steel sheet is pickled, cold rolling is
carried out on the steel sheet at a reduction in thickness of 40%
or more, the cold-rolled steel sheet is annealed at a maximum
temperature of 750.degree. C. to 900.degree. C., cooled to
450.degree. C. or lower at an average cooling rate of 0.1.degree.
C./second to 200.degree. C./second, and held in a temperature range
of 300.degree. C. to 450.degree. C. for 1 second to 1000
seconds.
Meanwhile, a flowchart of the method of manufacturing the
high-strength steel sheet of the embodiment is shown in FIG. 2 for
easy of understanding. Meanwhile, the broken lines in the flowchart
indicate processes or manufacturing conditions that are selected
according to necessity.
Furthermore, coating may be appropriately carried out on at least
one surface of the hot-rolled steel sheet and the cold-rolled steel
sheet. For example, zinc-based coating such as coating using
galvanizing and galvannealing can be formed as a coating. The
zinc-based coating can also be formed through electroplating or hot
dipping. The galvannealing coating can be obtained by, for example,
alloying a zinc coating (galvanizing coating) that is formed
through electroplating or hot dipping in a predetermined
temperature (for example, a temperature of 450.degree. C. to
600.degree. C., and a time of 10 seconds to 90 seconds). A
galvanizing steel sheet and a galvannealed steel sheet can be
manufactured as final products in the above manner.
Additionally, a variety of organic films and coatings can be formed
on the hot-rolled steel sheet, the cold-rolled steel sheet, the
galvanized steel sheet, and the galvannealed steel sheet.
EXAMPLES
Hereinafter, examples of the present invention will be
described.
Steels that had been prepared and melted in a converter and had the
chemical components as shown in Tables 1 to 3 were cast so as to
produce slabs. The steels having each chemical component were
heated to a temperature of 1150.degree. C. or higher in a heating
furnace, subjected to hot rolling at a finishing temperature of
850.degree. C. to 920.degree. C., cooled at an average cooling rate
of 30.degree. C./second, and coiled in a coiling temperature of
100.degree. C. to 600.degree. C., thereby producing 2.8 mm to 3.2
mm-thick hot-rolled steel sheets. The manufacturing conditions and
mechanical properties of the hot-rolled steel sheets are shown in
Tables 4 to 6, and the microstructures of the hot-rolled steel
sheets are shown in Tables 7 to 9.
TABLE-US-00001 TABLE I Chemical components (mass %) Acid- Acid-
Steel soluble soluble No. C Si Mn P S N Al Ti Cr Nb V Mo A1 0.067
0.48 1.9 0.015 0.0049 0.0033 0.024 -- -- -- -- -- A2 0.135 0.52 2.1
0.015 0.0030 0.0044 0.042 -- -- -- -- -- A3 0.068 0.60 2.5 0.020
0.0049 0.0037 0.034 -- -- -- -- -- A4 0.157 0.41 2.4 0.016 0.0029
0.0044 0.036 -- -- -- -- -- A5 0.153 1.15 2.2 0.008 0.0029 0.0006
0.030 0.006 0.5 0.01 -- -- A6 0.135 0.58 2.4 0.012 0.0038 0.0026
0.041 -- 0.4 -- -- 0.10 a1 0.070 0.52 1.9 0.015 0.0048 0.0036 0.026
-- -- -- -- -- a2 0.155 1.02 2.1 0.008 0.0031 0.0005 0.031 0.006
0.5 0.01 -- -- a3 0.072 0.62 2.4 0.020 0.0052 0.0035 0.034 -- -- --
-- -- a4 0.081 0.71 1.8 0.015 0.0015 0.0037 0.024 -- -- -- -- -- a5
0.083 0.62 2.3 0.016 0.0013 0.0026 0.003 0.005 -- 0.03 -- 0.15 a6
0.067 0.50 2.0 0.015 0.0029 0.0034 0.035 0.250 -- 0.15 -- -- a7
0.153 0.98 2.2 0.008 0.0029 0.0006 0.030 0.006 5.3 0.01 -- -- ([Ce]
+ [La]/) [Acid- ([Ce] | Steel Chemical components (mass %) soluble
[La])/ No. Zr B Ca Cu Ni Ce La Al] [S] A1 -- -- -- -- -- 0.0040 --
0.17 0.8 A2 -- -- 0.001 -- -- -- 0.0050 0.12 1.7 A3 -- -- -- -- --
-- 0.0040 0.12 0.8 A4 -- -- -- 0.1 -- -- 0.0050 0.14 1.7 A5 --
0.001 -- -- -- 0.0050 -- 0.17 1.7 A6 0.005 -- -- -- 0.05 -- 0.0060
0.15 1.6 a1 -- -- -- -- -- -- 0.0010 0.04 0.2 a2 -- 0.001 -- -- --
-- -- -- -- a3 -- -- -- -- -- -- -- -- -- a4 -- -- -- -- -- 0.0400
0.0400 3.32 53.3 a5 -- 0.002 -- -- -- 0.0300 0.0450 1.53 54.2 a6 --
-- -- -- -- 0.0020 0.0010 0.08 1.0 a7 -- 0.001 -- -- -- 0.0050 --
0.17 1.7 * "--" indicates that the corresponding chemical element
is not added. *The underlines in this Table indicate that the
corresponding amount does not satisfy the conditions of the
chemical components according to the present invention.
TABLE-US-00002 TABLE 2 Chemical components (mass %) Steel
Acid-soluble Acid-soluble No. C Si Mn P S N Al Ti Cr Nb V B1 0.07
0.42 1.9 0.014 0.0015 0.0034 0.025 0.02 -- -- -- B2 0.07 0.50 2.1
0.015 0.0030 0.0036 0.034 0.05 -- -- -- B3 0.14 0.49 2.0 0.015
0.0031 0.0047 0.041 0.02 -- -- -- B4 0.07 0.58 2.5 0.020 0.0051
0.0034 0.035 0.10 -- -- -- B5 0.08 0.59 2.3 0.015 0.0049 0.0033
0.042 0.02 -- -- -- B6 0.15 0.49 2.6 0.009 0.0010 0.0045 0.036 0.01
-- -- -- B7 0.16 2.07 2.0 0.010 0.0024 0.0022 0.031 0.02 -- 0.02 --
B8 0.15 1.03 2.0 0.008 0.0030 0.0006 0.031 0.01 0.5 0.01 -- B9 0.15
0.61 2.8 0.012 0.0042 0.0023 0.042 0.01 0.4 -- -- b1 0.07 0.49 2.0
0.016 0.0029 0.0034 0.036 0.05 -- -- -- b2 0.13 0.52 2.2 0.015
0.0030 0.0046 0.040 0.02 -- -- -- b3 0.08 0.59 2.3 0.015 0.0048
0.0037 0.039 0.02 -- -- -- b4 0.08 0.59 2.1 0.015 0.0029 0.0024
0.030 0.05 -- 0.03 -- b5 0.15 1.97 1.9 0.010 0.0026 0.0018 0.030
0.02 -- 0.02 -- b6 0.14 0.59 2.7 0.011 0.0038 0.0027 0.041 0.01 0.4
-- -- b7 0.07 0.38 1.8 0.015 0.0015 0.0037 0.025 0.02 -- -- -- b8
0.16 0.49 2.5 0.009 0.0010 0.0046 0.035 0.01 -- -- -- b9 0.35 0.62
3.6 0.012 0.0039 0.0025 0.041 -- 0.4 -- -- b10 0.07 0.50 2.0 0.015
0.0029 0.0034 0.035 0.25 -- 0.15 -- b11 0.15 0.98 2.2 0.008 0.0029
0.0006 0.030 0.02 5.3 0.01 -- Chemical components (mass %) ([Ce] +
[La])/ ([Ce] + Steel [Acid-soluble [La])/ No. Mo Zr B Ca Cu Ni Ce
La Al] [S] B1 -- -- -- -- -- -- 0.004 -- 0.16 2.67 B2 -- -- -- --
-- -- 0.006 -- 0.18 2.00 B3 -- -- -- 0.001 -- -- -- 0.005 0.12 1.61
B4 -- -- -- -- -- -- -- 0.004 0.11 0.78 B5 -- -- -- -- -- 0.05
0.002 0.002 0.10 0.82 B6 -- -- -- -- -- -- 0.001 0.003 0.11 4.03 B7
-- -- 0.001 -- -- -- 0.002 0.004 0.20 2.46 B8 -- -- 0.001 -- -- --
0.005 -- 0.16 1.67 B9 0.10 -- -- -- -- 0.05 -- 0.006 0.14 1.44 b1
-- -- -- -- -- -- -- 0.001 0.02 0.28 b2 -- -- -- 0.001 -- -- -- --
0.00 0.00 b3 -- -- -- -- -- 0.05 -- -- 0.00 0.00 b4 0.15 -- 0.002
-- -- -- 0.001 -- 0.03 0.34 b5 -- -- 0.001 -- -- -- -- -- 0.00 0.00
b6 0.10 0.005 -- -- -- 0.05 -- -- 0.00 0.00 b7 -- -- -- -- -- --
0.040 0.040 3.19 53.33 b8 -- -- -- -- -- -- 0.035 0.030 1.88 64.01
b9 0.10 0.005 -- -- -- 0.05 0.002 0.002 0.09 0.90 b10 -- -- -- --
-- -- 0.002 0.001 0.08 1.04 b11 -- -- 0.001 -- -- -- 0.005 0.000
1.14 1.74 * "--" indicates that the corresponding chemical element
is not added. *The underlines in this Table indicate that the
corresponding amount does not satisfy the conditions of the
chemical components according to the present invention.
TABLE-US-00003 TABLE 3 Chemical components (mass %) Steel
Acid-soluble Acid-soluble No. C Si Mn P S N Al Ti Cr Nb V C1 0.040
0.42 1.8 0.015 0.0029 0.0027 0.042 0.050 -- 0.01 -- C2 0.110 0.92
2.2 0.015 0.0025 0.0035 0.039 0.002 -- -- -- C3 0.165 1.45 2.5
0.008 0.0029 0.0025 0.040 0.004 -- 0.01 -- C4 0.130 1.00 2.2 0.010
0.0002 0.0036 0.030 0.010 -- 0.03 0.05 C5 0.060 0.70 2.0 0.010
0.0072 0.0035 0.039 0.040 -- -- -- C6 0.161 1.20 2.8 0.010 0.0038
0.0035 0.040 0.004 -- 0.02 -- C7 0.110 1.10 2.1 0.012 0.0035 0.0034
0.038 0.002 -- -- -- C8 0.080 0.87 1.5 0.009 0.0004 0.0033 0.032 --
-- -- -- C9 0.080 0.60 2.0 0.011 0.0105 0.0035 0.103 0.020 -- 0.02
-- C10 0.190 1.70 2.5 0.010 0.0420 0.0040 0.105 0.006 0.6 -- -- c1
0.250 0.62 4.2 0.012 0.0039 0.0025 0.041 -- 0.4 -- -- c2 0.110 0.05
2.2 0.010 0.0040 0.0036 1.900 0.010 -- -- -- c3 0.330 1.02 1.8
0.012 0.0035 0.0025 0.032 -- -- -- -- ([Ce] + [La])/ ([Ce] + Steel
Chemical components (mass %) [Acid-soluble [La])/ No. Mo Zr B Mg W
Ni Others Ce La Al] [S] C1 0.05 -- -- -- 0.4 -- -- 0.0010 0.0020
0.07 1.04 C2 -- -- -- -- -- -- -- 0.0013 0.0024 0.09 1.48 C3 -- --
0.001 0.004 -- -- -- 0.0025 -- 0.06 0.87 C4 -- -- -- -- -- -- --
0.0015 0.0010 0.08 12.50 C5 -- -- -- -- -- -- As:0.02, 0.0010
0.0020 0.08 0.42 Co:0.02, Sm:0.002 C6 -- -- -- -- -- -- Sn:0.04,
0.0030 -- 0.08 0.79 Pb:0.05 C7 -- -- -- -- -- -- Dy:0.003, 0.0015
0.0020 0.09 1.00 Nd:0.003 C8 -- -- -- -- -- -- Y:0.002, -- 0.0022
0.07 5.50 Hf:0.0025 C9 -- -- -- -- -- -- -- 0.0100 0.0120 0.12 1.14
C10 -- -- -- -- -- -- -- 0.0150 0.0210 0.20 0.50 c1 0.10 0.005 --
-- -- 0.05 -- 0.0015 0.0020 0.09 0.90 c2 -- -- -- -- -- -- --
0.0090 0.0010 0.01 2.50 c3 -- -- -- -- -- -- -- 0.0020 0.0025 0.08
1.29 * "--" indicates that the corresponding chemical element is
not added. *The underlines in this Table indicate that the
corresponding amount does not satisfy the conditions of the
chemical components according to the present invention.
TABLE-US-00004 TABLE 4 Hot-rolled conditions Mechanical properties
Steel Heating Finishing Coiling Tensile Elongation Hole sheet Steel
temperature temperature temperature strength El expansion No. No.
.degree. C. .degree. C. .degree. C. TS MPA % .lamda. % TS .times.
El .times. .lamda. A1-h1 A1 1180 900 350 572 30.2 94 1.6E+06 A1-h2
A1 1160 890 180 645 28.2 51 9.3E+05 A2-h1 A2 1180 900 360 745 23.4
76 1.3E+06 A2-h2 A2 1170 880 110 802 20.8 38 6.3E+05 A3-h1 A3 1200
890 380 720 24.5 79 1.4E+06 A3-h2 A3 1170 900 100 813 21.2 33
5.7E+05 A4-h1 A4 1150 880 330 932 17.8 67 1.1E+06 A4-h2 A4 1180 870
180 1023 16.1 31 5.1E+05 A5-h1 A5 1190 880 400 1072 14.6 62 9.6E+05
A5-h2 A5 1170 900 150 1196 15.6 21 3.9E+05 A6-h1 A6 1200 890 330
1068 15.3 65 1.1E+06 A6-h2 A6 1180 900 130 1236 14.2 23 4.0E+05
a1-h1 a1 1180 900 360 569 30.1 65 1.1E+06 a2-h1 a2 1200 890 410
1098 14.8 42 6.8E+05 a3-h1 a3 1160 870 400 725 24.2 54 9.5E+05
a4-h1 a4 1190 860 340 562 31.2 62 1.1E+06 a5-h1 a5 1210 900 370 932
18.2 45 7.6E+05 a6-h1 a6 1250 910 320 921 8.8 45 3.6E+05 a7-h1 a7
1200 880 350 1320 7.2 55 5.2E+05 * The underlines in the Table
indicate that the corresponding cell does not satisfy the
manufacturing conditions according to the present invention.
TABLE-US-00005 TABLE 5 Hot-rolled conditions Mechanical properties
Steel Heating Finishing Coiling Tensile Elongation Hole sheet Steel
temperature temperature temperature strength El expansion No. No.
.degree. C. .degree. C. .degree. C. TS MPA % .lamda. % TS .times.
El .times. .lamda. B1-h1 B1 1250 900 360 575 30.8 95 1.7E+06 B1-h2
B1 1250 890 180 643 28.2 55 1.0E+06 B2-h1 B2 1250 900 360 531 32.3
108 1.9E+06 B2-h2 B2 1250 880 110 646 26.8 51 8.8E+05 B3-h1 B3 1250
880 330 760 22.6 78 1.3E+06 B3-h2 B3 1250 870 180 837 19.0 42
6.7E+05 B4-h1 B4 1250 900 390 777 22.7 77 1.4E+06 B4-h2 B4 1250 880
150 835 19.9 40 6.7E+05 B5-h1 B5 1250 900 310 783 21.4 73 1.2E+06
B5-h2 B5 1250 810 100 845 18.2 42 6.5E+05 B6-h1 B6 1250 890 330 964
15.7 57 8.7E+05 B6-h2 B6 1250 900 180 1086 15.1 35 5.7E+05 B7-h1 B7
1250 880 350 1075 14.1 52 7.9E+05 B7-h2 B7 1250 890 150 1199 12.8
38 5.8E+05 B8-h1 B8 1210 870 370 1062 14.0 59 8.8E+05 B8-h2 B8 1200
880 180 1250 14.2 35 6.2E+05 B9-h1 B9 1210 900 390 1156 14.1 48
7.8E+05 B9-h2 B9 1210 880 160 1235 12.9 32 5.1E+05 b1-h1 b1 1250
890 360 533 33.1 76 1.33E+06 b2-h1 b2 1250 870 330 754 22.7 55
9.33E+05 b3-h1 b3 1250 910 310 777 21.3 51 8.43E+05 b4-h1 b4 1250
900 360 950 16.8 41 6.58E+05 b5-h1 b5 1250 880 350 1070 15.2 36
5.92E+05 b6-h1 b6 1210 870 370 1053 13.8 41 6.00E+05 b7-h1 b7 1250
900 360 574 31.3 67 1.20E+06 b8-h1 b8 1250 890 330 954 15.7 40
5.97E+05 b9-h1 b9 1250 880 350 1170 8.1 32 3.05E+05 b10-h1 b10 1250
880 320 905 9.1 45 3.72E+05 b11-h1 b11 1250 890 350 1313 7.1 50
4.65E+05 * The underlines in the Table indicate that the
corresponding cell does not satisfy the manufacturing conditions
according to the present invention.
TABLE-US-00006 TABLE 6 Hot-rolled conditions Mechanical properties
Steel Heating Finishing Coiling Tensile Elongation Hole sheet Steel
temperature temperature temperature strength El expansion No. No.
.degree. C. .degree. C. .degree. C. TS MPA % .lamda. % TS .times.
El .times. .lamda. C1-h1 C1 1250 920 380 552 31.2 123 2.1E+06 C1-h2
C1 1250 910 150 623 29.2 61 1.1E+06 C2-h1 C2 1200 890 350 983 16.6
76 1.2E+06 C2-h2 C2 1200 900 180 1092 15.3 39 6.5E+05 C3-h1 C3 1250
950 400 1176 14.9 65 1.1E+06 C3-h2 C3 1250 940 180 1265 13.8 31
5.4E+05 C4-h1 C4 1250 910 450 892 19.2 81 1.4E+06 C4-h2 C4 1250 930
160 1024 17.6 38 6.8E+05 C5-h1 C5 1200 880 350 621 27.8 121 2.1E+06
C5-h2 C5 1200 880 180 644 28.4 58 1.1E+06 C6-h1 C6 1250 880 380
1206 14.2 68 1.2E+06 C6-h2 C6 1250 890 150 1289 12.4 28 4.5E+05
C7-h1 C7 1200 900 400 945 18.6 76 1.3E+06 C7-h2 C7 1200 920 180
1056 16.2 36 6.2E+05 C8-h1 C8 1250 880 330 561 32.6 119 2.2E+06
C8-h2 C8 1250 890 160 603 30.1 67 1.2E+06 C9-h1 C9 1250 930 300 702
26.8 102 1.9E+06 C9-h2 C9 1250 930 150 791 24.1 42 8.0E+05 C10-h1
C10 1200 880 320 1191 16.3 78 1.5E+06 C10-h2 C10 1200 880 130 1253
13.4 21 3.5E+05 c1-h1 c1 1150 880 380 1024 9.3 35 3.3E+05 c2-h1 c2
1200 900 350 989 18.2 32 5.8E+05 c3-h1 c3 1200 920 380 773 9.3 31
3.3E+05 * The underlines in the Table indicate that the
corresponding cell does not satisfy the manufacturing conditions
according to the present invention.
TABLE-US-00007 TABLE 7 Elongated inclusions Inclusions including
sulfides Volume Average Volume Fine inclusions number equivalent
number Average Martensite phase Steel Area number Number density
circle Number density concentration Maximum sheet Steel density
percentage inclusion/ diameter percentage inclusion/ o- f [Ce] +
[La] Ratio hardness No. No. inclusion/mm.sup.2 % mm.sup.3 .mu.m %
mm.sup.3 % % HV A1-h1 A1 42 0 0 6 83 2.5 .times. 10.sup.4 34 10.2
487 A1-h2 A1 41 1 0 6 82 2.8 .times. 10.sup.4 38 10.4 643 A2-h1 A2
42 0 0 5 84 2.4 .times. 10.sup.4 40 18.2 516 A2-h2 A2 41 1 0 6 83
2.8 .times. 10.sup.4 28 16.4 665 A3-h1 A3 38 1 0 9 81 2.3 .times.
10.sup.4 31 16.2 534 A3-h2 A3 42 0 0 9 90 2.5 .times. 10.sup.4 33
16.4 681 A4-h1 A4 36 1 0 5 88 2.1 .times. 10.sup.4 28 26.1 549
A4-h2 A4 37 1 0 7 92 2.8 .times. 10.sup.4 40 25.3 668 A5-h1 A5 37 0
0 5 88 2.1 .times. 10.sup.4 37 33.2 546 A5-h2 A5 35 1 0 8 91 2.9
.times. 10.sup.4 28 34.6 630 A6-h1 A6 45 0 0 7 82 2.7 .times.
10.sup.4 25 35.6 511 A6-h2 A6 46 1 0 7 83 2.5 .times. 10.sup.4 31
34.6 657 a1-h1 a1 7 45 2.1 .times. 10.sup.4 29 3 5.0 .times.
10.sup.2 2 10.1 484 a2-h1 a2 9 32 2.8 .times. 10.sup.4 17 0 0 0
32.8 590 a3-h1 a3 6 41 3.0 .times. 10.sup.4 26 1 0 0 16.5 553 a4-h1
a4 52 0 0 7 89 2.1 .times. 10.sup.4 42 10.7 447 a5-h1 a5 49 1 0 5
92 3.0 .times. 10.sup.4 53 29.8 510 a6-h1 a6 63 0 0 9 84 2.0
.times. 10.sup.4 18 31.5 479 a7-h1 a7 33 0 0 7 88 2.5 .times.
10.sup.4 32 56.2 475
TABLE-US-00008 TABLE 8 Elongated inclusions Inclusions including
sulfides Volume Average Volume Fine inclusions number equivalent
number Average Martensite phase Steel Area number Number density
circle Number density concentration Maximum sheet Steel density
percentage inclusion/ diameter percentage inclusion/ o- f [Ce] +
[La] Ratio hardness No. No. inclusion/mm.sup.2 % mm.sup.3 .mu.m %
mm.sup.3 % % HV B1-h1 B1 42 0 0 6 82 2.1 .times. 10.sup.4 34 9.4
514 B1-h2 B1 41 1 0 7 81 2.8 .times. 10.sup.4 36 9.5 685 B2-h1 B2
38 0 0 4 83 2.3 .times. 10.sup.4 40 9.3 420 B2-h2 B2 37 0 0 8 81
2.6 .times. 10.sup.4 42 10.3 657 B3-h1 B3 37 0 0 6 81 2.5 .times.
10.sup.4 28 19.1 532 B3-h2 B3 40 1 0 6 80 2.6 .times. 10.sup.4 30
17.6 664 B4-h1 B4 39 0 0 7 83 2.4 .times. 10.sup.4 41 19.4 539
B4-h2 B4 38 0 0 8 84 2.5 .times. 10.sup.4 39 19.8 613 B5-h1 B5 41 1
0 5 81 2.6 .times. 10.sup.4 35 20.9 518 B5-h2 B5 42 0 0 6 82 2.4
.times. 10.sup.4 37 18.7 656 B6-h1 B6 41 1 0 6 81 2.4 .times.
10.sup.4 43 28.2 556 B6-h2 B6 39 0 0 5 80 2.1 .times. 10.sup.4 42
29.7 627 B7-h1 B7 39 0 0 6 81 2.5 .times. 10.sup.4 31 39.9 491
B7-h2 B7 41 0 0 7 80 2.0 .times. 10.sup.4 33 36.8 607 B8-h1 B8 38 0
0 9 80 2.1 .times. 10.sup.4 42 43.2 450 B8-h2 B8 42 1 0 7 80 2.5
.times. 10.sup.4 43 37.4 624 B9-h1 B9 39 0 0 6 83 2.3 .times.
10.sup.4 40 40.8 523 B9-h2 B9 36 0 0 5 81 2.8 .times. 10.sup.4 39
37.4 618 b1-h1 b1 5 45 2.1 .times. 10.sup.4 29 0 5.0 .times.
10.sup.2 1 9.2 423 b2-h1 b2 7 32 2.8 .times. 10.sup.4 17 0 0 0 18.9
537 b3-h1 b3 6 41 3.0 .times. 10.sup.4 26 0 0 0 20.4 528 b4-h1 b4 4
40 2.3 .times. 10.sup.4 25 0 5.0 .times. 10.sup.2 1 29.3 531 b5-h1
b5 6 42 2.7 .times. 10.sup.4 24 0 0 0 38.9 502 b6-h1 b6 4 40 3.0
.times. 10.sup.4 23 0 0 0 42.2 458 b7-h1 b7 55 1 0 6 89 2.1 .times.
10.sup.4 68 9.0 534 b8-h1 b8 63 2 0 5 91 3.0 .times. 10.sup.4 72
27.5 568 b9-h1 b9 34 0 0 6 81 1.8 .times. 10.sup.4 21 59.9 397
b10-h1 b10 41 0 0 9 84 2.0 .times. 10.sup.4 18 29.2 494 b11-h1 b11
33 0 0 7 88 2.5 .times. 10.sup.4 32 49.8 515
TABLE-US-00009 TABLE 9 Elongated inclusions Inclusions including
sulfides Volume Average Volume Fine inclusions number equivalent
number Average Martensite phase Steel Area number Number density
circle Number density concentration Maximum sheet Steel density
percentage inclusion/ diameter percentage inclusion/ o- f [Ce] +
[La] Ratio hardness No. No. inclusion/mm.sup.2 % mm.sup.3 .mu.m %
mm.sup.3 % % HV C1-h1 C1 42 1 0 7 82 2.6 .times. 10.sup.4 32 2.5
580 C1-h2 C1 45 0 0 8 80 2.0 .times. 10.sup.4 34 3 725 C2-h1 C2 38
0 0 5 83 2.1 .times. 10.sup.4 33 29 515 C2-h2 C2 36 1 0 6 85 2.2
.times. 10.sup.4 35 32 656 C3-h1 C3 42 0 0 7 80 2.0 .times.
10.sup.4 36 42 536 C3-h2 C3 48 0 0 7 84 2.3 .times. 10.sup.4 39 39
672 C4-h1 C4 25 1 0 6 81 2.5 .times. 10.sup.4 35 28 492 C4-h2 C4 27
1 0 7 80 2.2 .times. 10.sup.4 37 26 648 C5-h1 C5 41 1 0 7 81 2.0
.times. 10.sup.4 34 5.5 550 C5-h2 C5 44 1 0 8 80 2.5 .times.
10.sup.4 31 6 783 C6-h1 C6 40 0 0 8 81 2.1 .times. 10.sup.4 32 45
563 C6-h2 C6 46 0 0 7 83 2.7 .times. 10.sup.4 36 42 692 C7-h1 C7 27
0 0 6 80 2.2 .times. 10.sup.4 34 25 521 C7-h2 C7 28 0 0 6 82 2.3
.times. 10.sup.4 36 27 666 C8-h1 C8 21 0 0 6 88 1.6 .times.
10.sup.4 42 6 569 C8-h2 C8 22 0 0 7 85 1.7 .times. 10.sup.4 39 7
702 C9-h1 C9 76 1 0 8 81 4.3 .times. 10.sup.4 25 13 563 C9-h2 C9 82
1 0 7 76 3.9 .times. 10.sup.4 27 14 673 C10-h1 C10 103 1 1.0
.times. 10.sup.3 8 79 6.2 .times. 10.sup.4 24 42 562 C10-h2 C10 111
1 1.0 .times. 10.sup.3 9 83 5.8 .times. 10.sup.4 25 41 715 c1-h1 c1
25 2 1.0 .times. 10.sup.3 7 85 1.3 .times. 10.sup.4 18 48 556 c2-h1
c2 10 5 3.0 .times. 10.sup.3 9 8 5.0 .times. 10.sup.2 5 25 582
c3-h1 c3 26 1 1.0 .times. 10.sup.3 8 82 1.5 .times. 10.sup.4 21 26
571
With regard to cold-rolled steel sheets, firstly, steels having the
above chemical compositions were cast, heated to 1150.degree. C. or
higher, subjected to hot rolling in a finishing temperature of
850.degree. C. to 910.degree. C., cooled at an average cooling rate
of 30.degree. C./second, and coiled at a coiling temperature of
450.degree. C. to 610.degree. C., thereby producing 2.8 mm to 3.2
mm-thick hot-rolled steel sheets. After that, the hot-rolled steel
sheets were pickled, and then subjected to cold rolling, annealing,
and holding under the conditions as shown in Tables 10 to 12,
thereby producing cold-rolled steel sheets. The manufacturing
conditions and mechanical properties of the cold-rolled steel
sheets are shown in Tables 10 to 12 and the microstructures of the
cold-rolled steel sheets are shown in Tables 13 to 15. The sheet
thicknesses of the cold-rolled steel sheets were 0.5 mm to 2.4
mm.
TABLE-US-00010 TABLE 10 Cold-rolled conditions Mechanical
properties Hot-rolled conditions Annea- Hole Heating Finishing
Coiling ling Average Holding Sheet Tensile Elonga- e- xpan- Steel
temper- temper- temper- Reduc- temper- cooling temper- Holding
thic- k- strength tion sion sheet Steel ature ature ature tion
ature rate ature time ness TS El .lamda- . No. No. .degree. C.
.degree. C. .degree. C. % .degree. C. .degree. C. .degree. C. s mm
MPa % % TS .times. El .times. .lamda. A1-c1 A1 1180 900 600 55 790
14 350 330 0.8 562 32.2 102 1.8E+06 A1-c2 A1 1160 890 580 55 790 18
250 330 0.8 662 28.2 48 9.0E+05 A2-c1 A2 1180 900 590 55 810 14 380
300 1.6 722 24.3 75 1.3E+06 A2-c2 A2 1170 880 610 55 810 17 280 300
1.6 791 20.3 38 6.1E+05 A3-c1 A3 1200 890 500 60 810 15 340 320 1.2
720 22.0 81 1.3E+06 A3-c2 A3 1170 900 510 60 810 19 240 320 1.2 813
19.8 35 5.6E+05 A4-c1 A4 1150 880 450 60 830 16 350 320 1.4 945
17.5 71 1.2E+06 A4-c2 A4 1180 870 480 60 830 19 260 320 1.4 1032
15.9 28 4.6E+05 A5-c1 A5 1190 880 560 50 800 14 380 350 1.2 1082
13.2 59 8.4E+05 A5-c2 A5 1170 900 570 50 800 17 280 350 1.2 1203
13.9 19 3.2E+05 A6-c1 A6 1200 890 540 60 810 15 350 350 0.6 1072
15.8 67 1.1E+06 A6-c2 A6 1180 900 520 60 810 18 250 350 0.6 1251
13.1 17 2.8E+05 a1-c1 a1 1180 900 590 55 810 15 350 330 2.1 572
30.1 59 1.0E+06 a2-c1 a2 1200 890 550 50 800 14 380 350 0.9 1075
13.1 43 6.1E+05 a3-c1 a3 1160 870 490 60 810 15 340 320 1.5 725
23.8 51 8.8E+05 a4-c1 a4 1190 850 400 55 810 13 400 300 2.3 557
33.2 67 1.2E+06 a5-c1 a5 1210 900 520 60 850 15 340 300 1.6 941
18.9 44 7.8E+05 a6-c1 a6 1250 910 570 55 800 14 380 300 1.4 932 8.2
47 3.6E+05 a7-c1 a7 1200 880 570 50 800 14 380 350 0.7 1280 7.3 51
4.8E+05 * The underlines in the Table indicate that the
corresponding cell does not satisfy the manufacturing conditions
according to the present invention.
TABLE-US-00011 TABLE 11 Cold-rolled conditions Mechanical
properties Hot-rolled conditions Annea Hole Heating Finishing
Coiling ling- Average Holding Sheet Tensile Elonga- - expan- Steel
temper- temper- temper- Reduc- temper- cooling temper- Holding
thic- k- strength tion sion sheet Steel ature ature ature tion
ature rate ature time ness TS El .lamda- . No. No. .degree. C.
.degree. C. .degree. C. % .degree. C. .degree. C. .degree. C. s mm
MPa % % TS .times. El .times. .lamda. B1-c1 B1 1250 900 500 60 810
15 350 330 2.1 547 31.5 105 1.8E+06 B1-c2 B1 1250 900 510 60 810 18
250 330 2.1 623 28.8 61 1.1E+06 B2-c1 B2 1250 910 580 55 870 16 380
300 0.7 535 33.1 115 2.0E+06 B2-c2 B2 1250 910 600 55 870 19 280
300 0.7 658 27.8 55 1.0E+06 B3-c1 B3 1250 880 490 60 790 15 330 360
1.0 785 22.1 78 1.4E+06 B3-c2 B3 1250 890 480 60 790 17 260 360 1.0
804 20.6 63 1.0E+06 B4-c1 B4 1250 900 560 55 800 13 400 360 0.8 759
22.9 85 1.5E+06 B4-c2 B4 1250 900 550 55 800 20 200 360 0.8 853
19.8 58 9.8E+05 B5-c1 B5 1250 910 590 50 810 16 320 330 0.5 763
23.4 88 1.6E+06 B5-c2 B5 1250 900 600 50 810 18 250 330 0.5 841
19.2 56 9.0E+05 B6-c1 B6 1250 900 580 60 850 15 380 330 1.2 916
18.2 75 1.3E+06 B6-c2 B6 1250 900 570 60 850 20 250 330 1.2 1074
15.0 36 5.8E+05 B7-c1 B7 1250 900 580 60 800 14 380 250 1.2 1086
14.3 58 9.0E+05 B7-c2 B7 1250 900 590 60 800 18 250 250 1.2 1254
12.8 28 4.5E+05 B8-c1 B8 1220 900 610 55 820 17 310 300 1.4 1036
14.7 63 9.6E+05 B8-c2 B8 1210 890 590 55 820 18 280 300 1.4 1251
14.5 31 5.6E+05 B9-c1 B9 1210 890 550 50 820 15 350 330 2.0 1132
14.4 54 8.8E+05 B9-c2 B9 1220 880 540 50 820 19 250 330 2.0 1207
13.9 29 4.9E+05 b1-c1 b1 1250 900 590 55 870 16 380 300 1.8 524
32.4 79 1.3E+06 b2-c1 b2 1250 900 500 60 790 15 330 360 0.7 779
22.5 54 9.5E+05 b3-c1 b3 1250 890 600 50 810 16 320 330 0.9 771
22.2 61 1.0E+06 b4-c1 b4 1250 900 540 60 810 15 350 360 1.0 939
17.1 48 7.7E+05 b5-c1 b5 1250 890 600 60 800 14 380 250 1.4 1109
14.6 37 6.0E+05 b6-c1 b6 1200 910 600 55 820 17 310 300 0.6 1045
14.2 41 6.1E+05 b7-c1 b7 1250 900 510 60 810 15 380 330 1.2 554
31.1 72 1.2E+06 b8-c1 b8 1250 890 580 60 850 15 380 360 1.6 914
17.3 48 7.6E+05 b9-c1 b9 1250 900 500 55 810 15 350 350 0.6 1216
8.0 38 3.7E+05 b10-c1 b10 1250 910 550 55 850 16 350 300 1.1 887
8.8 48 3.7E+05 b11-c1 b11 1250 880 570 50 880 19 310 350 1.2 1347
7.0 31 2.9E+05 * The underlines in the Table indicate that the
corresponding cell does not satisfy the manufacturing conditions
according to the present invention.
TABLE-US-00012 TABLE 12 Cold-rolled conditions Mechanical
properties Hot-rolled conditions Annea- Hole Heating Finishing
Coiling ling Average Holding Sheet Tensile Elonga- e- xpan- Steel
temper- temper- temper- Reduc- temper- cooling temper- Holding
thic- k- strength tion sion sheet Steel ature ature ature tion
ature rate ature time ness TS El .lamda- . No. No. .degree. C.
.degree. C. .degree. C. % .degree. C. .degree. C. .degree. C. s mm
MPa % % TS .times. El .times. .lamda. C1-h1 C1 1250 900 500 45 810
15 350 330 1.6 572 31.1 129 2.3E+06 C1-h2 C1 1250 900 510 45 810 18
250 330 1.6 621 32.3 58 1.2E+06 C2-h1 C2 1200 880 550 60 850 14 300
300 1.3 983 16.8 76 1.3E+06 C2-h2 C2 1200 880 550 60 850 17 220 300
1.3 1092 15.6 39 6.6E+05 C3-h1 C3 1250 900 550 55 820 18 320 300
1.4 1176 15.5 65 1.2E+06 C3-h2 C3 1250 900 550 55 820 22 200 300
1.4 1265 14.5 31 5.7E+05 C4-h1 C4 1250 900 500 55 860 15 380 400
1.7 892 19.2 81 1.4E+06 C4-h2 C4 1250 900 500 55 860 18 250 400 1.7
1024 17.6 38 6.8E+05 C5-h1 C5 1200 880 600 80 820 16 350 600 0.8
601 30.1 121 2.2E+06 C5-h2 C5 1200 880 610 80 820 21 180 600 0.8
647 30.3 63 1.2E+06 C6-h1 C6 1250 900 350 70 850 16 350 300 1.2
1201 14.2 67 1.1E+06 C6-h2 C6 1250 900 360 70 850 21 250 300 1.2
1296 14.1 33 6.0E+05 C7-h1 C7 1200 910 430 60 820 17 400 600 1.0
946 19.1 82 1.5E+06 C7-h2 C7 1200 910 450 60 820 23 250 600 1.0
1079 17.5 41 7.7E+05 C8-h1 C8 1250 880 550 65 860 7 300 500 2.2 543
34.2 112 2.1E+06 C8-h2 C8 1250 890 530 65 860 9 220 500 2.2 631
29.6 65 1.2E+06 C9-h1 C9 1250 930 500 55 810 35 350 660 2.4 697
23.6 102 1.7E+06 C9-h2 C9 1250 930 510 55 810 41 250 660 2.4 746
22.5 48 8.1E+05 C10-h1 C10 1200 880 550 60 820 53 400 320 1.6 1201
15.8 74 1.4E+06 C10-h2 C10 1200 880 550 60 820 62 220 320 1.6 1291
14.1 23 4.2E+05 c1-h1 c1 1150 880 600 60 810 7 350 300 1.3 1281 8.5
31 3.4E+05 c2-h1 c2 1200 880 600 60 820 15 330 300 0.5 989 17.6 32
5.6E+05 c3-h1 c3 1200 930 550 55 810 8 330 450 1.9 811 9.5 33
5.6E+05 * The underlines in the Table indicate that the
corresponding cell does not satisfy the manufacturing conditions
according to the present invention.
TABLE-US-00013 TABLE 13 Elongated inclusions Inclusions including
sulfides Volume Average Volume Fine inclusions number equivalent
number Average Martensite phase Steel Area number Number density
circle Number density concentration Maximum sheet Steel density
percentage inclusion/ diameter percentage inclusion/ o- f [Ce] +
[La] Ratio hardness No. No. inclusion/mm.sup.2 % mm.sup.3 .mu.m %
mm.sup.3 % % HV A1-c1 A1 45 0 0 6 81 2.2 .times. 10.sup.4 37 11.2
431 A1-c2 A1 38 1 0 6 89 2.5 .times. 10.sup.4 41 9.8 717 A2-c1 A2
37 0 0 5 79 2.3 .times. 10.sup.4 43 17.3 528 A2-c2 A2 38 1 0 6 88
3.0 .times. 10.sup.4 29 16.8 636 A3-c1 A3 39 1 0 8 78 2.5 .times.
10.sup.4 32 19.5 476 A3-c2 A3 38 0 0 8 87 2.2 .times. 10.sup.4 33
17.9 632 A4-c1 A4 34 1 0 5 91 2.3 .times. 10.sup.4 27 30.2 514
A4-c2 A4 35 1 0 7 85 2.5 .times. 10.sup.4 39 28.3 614 A5-c1 A5 36 0
0 4 93 2.2 .times. 10.sup.4 40 37.8 514 A5-c2 A5 33 1 0 7 95 2.7
.times. 10.sup.4 29 34.4 638 A6-c1 A6 41 0 0 6 88 3.0 .times.
10.sup.4 23 36.6 522 A6-c2 A6 45 1 0 6 76 2.6 .times. 10.sup.4 34
35.2 657 a1-c1 a1 7 46 2.5 .times. 10.sup.4 26 3 5.0 .times.
10.sup.2 2 11.5 442 a2-c1 a2 8 32 2.4 .times. 10.sup.4 16 0 0 0
37.4 514 a3-c1 a3 5 41 3.1 .times. 10.sup.4 24 1 0 0 19 492 a4-c1
a4 49 0 0 6 97 2.3 .times. 10.sup.4 41 10.3 449 a5-c1 a5 50 1 0 5
96 3.1 .times. 10.sup.4 49 28.2 541 a6-c1 a6 57 0 0 8 78 2.1
.times. 10.sup.4 16 31.2 491 a7-c1 a7 32 0 0 7 92 2.9 .times.
10.sup.4 32 50.1 501
TABLE-US-00014 TABLE 14 Elongated inclusions Inclusions including
sulfides Volume Average Volume Fine inclusions number equivalent
number Average Martensite phase Steel Area number Number density
circle Number density concentration Maximum sheet Steel density
percentage inclusion/ diameter percentage inclusion/ o- f [Ce] +
[La] Ratio hardness No. No. inclusion/mm.sup.2 % mm.sup.3 .mu.m %
mm.sup.3 % % HV B1-c1 B1 38 1 0 8 82 2.1 .times. 10.sup.4 48 9.2
467 B1-c2 B1 39 0 0 7 73 2.9 .times. 10.sup.4 48 9.1 659 B2-c1 B2
40 0 0 7 88 2.2 .times. 10.sup.4 49 8.9 447 B2-c2 B2 35 1 0 6 78
2.1 .times. 10.sup.4 37 9.9 696 B3-c1 B3 36 0 0 6 85 2.6 .times.
10.sup.4 38 18.7 574 B3-c2 B3 38 0 0 5 83 2.0 .times. 10.sup.4 35
17.9 618 B4-c1 B4 42 1 0 6 90 2.9 .times. 10.sup.4 50 20.0 511
B4-c2 B4 35 1 0 5 85 2.8 .times. 10.sup.4 50 18.6 661 B5-c1 B5 41 0
0 5 81 2.3 .times. 10.sup.4 48 20.9 498 B5-c2 B5 42 0 0 5 74 2.3
.times. 10.sup.4 47 18.1 662 B6-c1 B6 41 0 0 6 83 2.3 .times.
10.sup.4 37 29.5 502 B6-c2 B6 40 0 0 5 85 2.7 .times. 10.sup.4 48
28.4 647 B7-c1 B7 39 0 0 6 74 2.8 .times. 10.sup.4 42 39.2 501
B7-c2 B7 41 1 0 6 80 2.1 .times. 10.sup.4 44 36.8 634 B8-c1 B8 38 0
0 5 81 2.6 .times. 10.sup.4 50 43.8 431 B8-c2 B8 45 0 0 8 86 2.3
.times. 10.sup.4 49 35.8 646 B9-c1 B9 42 0 0 7 76 2.6 .times.
10.sup.4 37 41.5 504 B9-c2 B9 36 0 0 7 81 2.8 .times. 10.sup.4 46
36.0 616 b1-c1 b1 4 43 2.5 .times. 10.sup.4 27 0 5.0 .times.
10.sup.2 1 8.7 428 b2-c1 b2 6 35 2.3 .times. 10.sup.4 21 0 0 0 18.5
570 b3-c1 b3 6 42 3.1 .times. 10.sup.4 24 0 0 0 21.2 499 b4-c1 b4 3
45 2.8 .times. 10.sup.4 25 0 5.0 .times. 10.sup.2 1 29.0 527 b5-c1
b5 6 40 2.1 .times. 10.sup.4 21 0 0 0 39.3 514 b6-c1 b6 3 39 2.3
.times. 10.sup.4 20 0 0 0 42.7 445 b7-c1 b7 50 1 0 5 92 2.2 .times.
10.sup.4 72 9.1 488 b8-c1 b8 62 2 0 6 92 2.1 .times. 10.sup.4 65
29.4 501 b9-c1 b9 31 0 0 7 89 2.0 .times. 10.sup.4 35 57.5 424
b10-c1 b10 38 0 0 8 82 2.5 .times. 10.sup.4 27 28.0 499 b11-c1 b11
36 0 0 6 84 2.6 .times. 10.sup.4 37 51.4 523
TABLE-US-00015 TABLE 15 Elongated inclusions Inclusions including
sulfides Volume Average Volume Fine inclusions number equivalent
number Average Martensite phase Steel Area number Number density
circle Number density concentration Maximum sheet Steel density
percentage inclusion/ diameter percentage inclusion/ o- f [Ce] +
[La] Ratio hardness No. No. inclusion/mm.sup.2 % mm.sup.3 .mu.m %
mm.sup.3 % % HV C1-h1 C1 41 0 0 8 79 2.0 .times. 10.sup.4 42 2.5
550 C1-h2 C1 46 1 0 8 82 2.4 .times. 10.sup.4 38 2 753 C2-h1 C2 37
1 0 5 84 2.3 .times. 10.sup.4 36 29 515 C2-h2 C2 36 1 0 6 81 2.5
.times. 10.sup.4 39 32 656 C3-h1 C3 43 0 0 6 83 2.6 .times.
10.sup.4 42 42 536 C3-h2 C3 46 0 0 5 81 2.1 .times. 10.sup.4 39 39
672 C4-h1 C4 27 1 0 7 85 2.2 .times. 10.sup.4 34 28 492 C4-h2 C4 29
1 0 8 82 2.2 .times. 10.sup.4 38 26 648 C5-h1 C5 41 0 0 6 81 2.3
.times. 10.sup.4 29 6 543 C5-h2 C5 44 0 0 6 80 2.1 .times. 10.sup.4
31 5.5 753 C6-h1 C6 44 0 0 7 82 2.1 .times. 10.sup.4 45 45 556
C6-h2 C6 45 0 0 8 80 2.3 .times. 10.sup.4 43 43 692 C7-h1 C7 29 0 0
6 84 2.5 .times. 10.sup.4 32 29 512 C7-h2 C7 31 0 0 7 83 2.7
.times. 10.sup.4 35 31 656 C8-h1 C8 23 1 0 7 87 1.7 .times.
10.sup.4 41 8 545 C8-h2 C8 21 1 0 8 86 1.8 .times. 10.sup.4 39 9
682 C9-h1 C9 78 1 0 8 82 4.2 .times. 10.sup.4 28 18 584 C9-h2 C9 81
1 0 7 77 3.6 .times. 10.sup.4 26 15 642 C10-h1 C10 102 0 1.0
.times. 10.sup.3 9 78 6.4 .times. 10.sup.4 22 42 578 C10-h2 C10 113
1 1.0 .times. 10.sup.3 9 82 5.9 .times. 10.sup.4 23 41 725 c1-h1 c1
26 2 0 8 82 1.3 .times. 10.sup.4 22 43 556 c2-h1 c2 11 1 3.0
.times. 10.sup.3 9 7 3.0 .times. 10.sup.3 5 25 582 c3-h1 c3 27 1
1.0 .times. 10.sup.3 7 85 1.9 .times. 10.sup.4 25 26 554
With regard to the elongated inclusions in the steel sheets, the
presence of coarse inclusions was confirmed using an optical
microscope, and the area number density of inclusions having an
equivalent circle diameter of 2 .mu.m or less with respect to
inclusions having an equivalent circle diameter of 0.5 .mu.m or
more was investigated through observation using a SEM. Even for
inclusions having an elongation ratio of 5 or more, the number
percentage, the volume number density, and the average equivalent
circle diameter were investigated.
Furthermore, with regard to unelongated inclusions in the steel
sheet, the number percentage and volume number density of
inclusions having MnS precipitated on oxides or oxysulfides (hard
compounds) including at least one of Ce and La with respect to
inclusions having an equivalent circle diameter of 1 .mu.m or more,
and the average value of the total amount of one or both of Ce and
La that are included in the inclusions were investigated.
The investigation results of inclusions in the hot-rolled steel
sheets are shown in Tables 7 to 9, and the investigation results of
inclusions in the cold-rolled steel sheets are shown in Tables 13
to 15. Meanwhile, in Tables 7 to 9 and Tables 13 to 15, fine
inclusions refer to inclusions having an equivalent circle diameter
of 0.5 .mu.m to 2 .mu.m, elongated inclusions refer to inclusions
having an equivalent circle diameter of 1 .mu.m or more and an
elongation ratio of 5 or more, and inclusions including sulfides
refer to inclusions that have MnS-based inclusions precipitated on
oxides or oxysulfides including at least one of Ce and La and have
an equivalent circle diameter of 1 .mu.m or more.
Firstly, the test results of manufacturing of hot-rolled steel
sheets will be described with reference to Tables 1 to 9.
In Steel sheet Nos. b9-h1 and c3-h1 in which Steel Nos. b9 and c3
are used, the concentration of C exceeds 0.3%. In Steel sheet No.
c1-h1 in which Steel No. c1 is used, the concentration of Mn
exceeds 4.0%. In Steel sheet Nos. a6-h1 and b10-h1 in which Steel
Nos. a6 and b10 are used, the concentration of the acid-soluble Ti
exceeds 0.20%. As a result, in Steel sheet Nos. b9-h1, c3-h1,
c1-h1, a6-h1, and b10-h1, elongation and hole expansion were
significantly small.
In addition, in Steel sheet No. c2-h1 in which Steel No. c2 was
used, the concentration of Si exceeded 2.1%, and
([Ce]+[La])/[acid-soluble Al] was less than 0.02, and therefore
hole expansion were small.
In Steel sheet Nos. a7-h1 and b11-h1 in which Steel Sheet Nos. a7
and b11 were used, the concentration of Cr exceeded 2.0%, and
therefore elongation was significantly small.
In Steel sheet Nos. a1-h1 to a5-h1 and b1-h1 to b8-h1 in which
Steel Nos. a1 to a5 and b1 to b8 were used, ([Ce]+[La])/[5] was
less than 0.4, or exceeded 50. Therefore, in the steel sheets, the
morphologies of inclusions were not sufficiently controlled, and
elongation and hole expansion degraded compared to steel sheets
having the same chemical composition except for Ce and La.
In Steel Nos. A1-h2 to A6-h2, B1-h2 to B9-h2, and C1-h2 to C10-h2
in which Steel sheet Nos. A1 to A6, B1 to B9, and C1 to C10 were
used, the coiling temperature was lower than 300.degree. C.
Therefore, in the above steel sheet Nos., the difference in
hardness between martensite and ferrite increased, and hole
expansion degraded compared to Steel sheet Nos. A1-h1 to A6-h1,
B1-h1 to B9-h1, and C1-h1 to C10-h1 having the same chemical
composition.
In Steel Nos. A1-h1 to A6-h1, B1-h1 to B9-h1, and C1-h1 to C10-h1
in which Steel sheet Nos. A1 to A6, B1 to B9, and C1 to C10 were
used, the morphologies of inclusions were sufficiently controlled,
and therefore elongation and hole expansion were sufficient.
Next, the test results of manufacturing of cold-rolled steel sheets
will be described with reference to Tables 1 to 3 and 10 to 15.
Similarly to the test results of manufacturing of hot-rolled steel
sheets, in Steel sheet Nos. a6-c1, a7-c1, b9-c1 to b11-c1, c1-c1 to
c3-c1 in which Steel Nos. a6, a7, b9 to b11, and c1 to c3 were
used, elongation or hole expansion were significantly small.
In addition, in Steel sheet Nos. a1-c1 to a5-c1 and b1-c1 to b8-c1
in which Steel Nos. a1 to a5 and b1 to b8 were used,
([Ce]+[La])/[S] was less than 0.4 or exceeded 50. Therefore, in the
steel sheets, the morphologies of inclusions were not sufficiently
controlled, and elongation and hole expansion degraded compared to
steel sheets having the same chemical composition except for Ce and
La.
In Steel Nos. A1-c2 to A6-c2, B1-c2 to B9-c2, and C1-c2 to C10-c2
in which Steel sheet Nos. A1 to A6, B1 to B9, and C1 to C10 were
used, the coiling temperature was lower than 300.degree. C.
Therefore, in the above steel sheet Nos., the difference in
hardness between martensite and ferrite increased, and hole
expansion degraded compared to Steel sheet Nos. A1-c1 to A6-c1,
B1-c1 to B9-c1, and C1-c1 to C10-c1 having the same chemical
composition.
In Steel sheet Nos. A1-c1 to A6-c1, B1-c1 to B9-c1, and C1-c1 to
C10-c1 in which Steel sheet Nos. A1 to A6, B1 to B9, and C1 to C10
were used, the morphologies of inclusions were sufficiently
controlled, and therefore elongation and hole expansion were
sufficient.
INDUSTRIAL APPLICABILITY
According to the present invention, since it is possible to obtain
a high-strength steel sheet that can be preferably mainly pressed
and used for underbody parts of automobiles and the like and
structural materials, and is excellent in terms of hole expansion
and ductility, the present invention significantly contributes to
steel industry, and has a large industrial availability.
* * * * *